Difference between revisions of "The Works of Francis Balfour 2-12"

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==Chapter XII. Chaetopoda==
 +
 +
Formation of the Germinal Layers.
 +
 +
MOST Chaetopoda deposit their eggs before development.
 +
The Oligochaeta lay them in peculiar cocoons or sacks formed
 +
by a secretion of the integument. Some marine Polychaeta carry
 +
them about during their development. Autolytus cornutus has
 +
a special sack on the ventral surface in which they are hatched.
 +
In Spirorbis Pagenstecheri they develop inside the opercular
 +
tentacle, and in Spirorbis spirillum inside the tube of the
 +
parent.
 +
 +
A few forms (e.g. Eunice sanguinea, Syllis vivipara, Nereis
 +
diversicolor) are viviparous.
 +
 +
Perhaps the most primitive type of Chaetopod development
 +
so far observed is that of Serpula (Stossich, No. 357) 2 . There is
 +
a regular segmentation resulting in the formation of a blastosphere with a central segmentation cavity. An invagination of
 +
the normal type now ensues. The blastopore soon narrows to
 +
become the permanent anus, while the invaginated hypoblast
 +
forms a small prominence with an imperfectly developed lumen,
 +
which does not nearly fill up the segmentation cavity (fig. 139 A).
 +
The embryo, which has in the meantime become completely
 +
 +
1 The following classification of the Chaetopoda is adopted in the present section.
 +
 +
I. Achseta (Polygordius).
 +
 +
(Sedentaria.
 +
 +
ii. Polychseta. j Errantia
 +
ill. Oligochgeta.
 +
 +
2 The observations of Stossich are not thoroughly satisfactory.
 +
 +
 +
 +
320 FORMATION OF THE LAYERS.
 +
 +
covered with cilia, now assumes more or less the form of a cone,
 +
at the apex of which is the anus, while the base forms the
 +
rudiment of a large prae-oral lobe. The alimentary sack grows
 +
forwards and then bends upon itself nearly at right angles, and
 +
meets a stomodaeal invagination from the ventral side some way
 +
from the front end of the body.
 +
 +
The alimentary canal soon differentiates itself into three
 +
regions (i) oesophagus, (2) stomach, and (3) intestine. With
 +
 +
 +
 +
 +
FlG. 139. TWO STAGES IN THE DEVELOPMENT OF SfiRPULA. (After StOSsich.)
 +
 +
m. mouth ; an. anus ; al. archenteron.
 +
 +
these changes the larva, which in the meantime becomes hatched,
 +
assumes the characters of a typical Annelid larva (fig. 139 B).
 +
In front is a large prae-oral lobe, at the sides of which the eyespots soon appear. The primitive segmentation cavity remains
 +
as a wide space between the curved alimentary tract and the
 +
body walls, and becomes traversed by muscular fibres passing
 +
between the two. The original chorion appears to serve as
 +
cuticle, and is perforated by the cilia.
 +
 +
The further changes in this larval form do not present features of
 +
general importance. A peculiar vesicle, which in anomalous cases is
 +
double, is formed near the anus. If it were shewn to occur widely
 +
amongst Chaetopoda, it might be perhaps regarded as homologous with
 +
the anal vesicles of the Gephyrea.
 +
 +
Serpula is one of the few Chaetopoda at present known in
 +
 +
 +
 +
CH^TOPODA. 321
 +
 +
 +
 +
which the segmentation is quite regular 1 . In other forms it is
 +
more or less unequal. The formation of the germinal layers
 +
has been far more fully studied in the Oligochaeta than in the
 +
Polychaeta, and though unfortunately the development is much
 +
abbreviated in the former group, they nevertheless have to serve
 +
as our type ; and unless the contrary is indicated the statements
 +
in the remainder of the section apply to the Oligochaeta. The
 +
segmentation is nearly regular in Lumbricus agricola (Kowalevsky) and results in the formation of a flattened blastosphere,
 +
one of the sides of which is hypoblastic and the other epiblastic,
 +
the hypoblast cells being easily distinguished from the epiblast
 +
cells by their clearer aspect. An invagination takes place,
 +
in the course of which the hypoblast becomes enclosed by the
 +
epiblast, and a somewhat cylindrical two-layered gastrula is
 +
formed. The opening of this gastrula at first extends over
 +
the whole of what becomes the ventral surface of the future
 +
worm, but gradually narrows to a small pore the permanent
 +
mouth near the front end. The central cavity of the gastrula is lined by hypoblast cells, but the oral opening, which
 +
leads by a narrow passage into the gastric cavity, is lined by
 +
epiblast cells.
 +
 +
The segmentation of Lumbricus trapezoides (Kleinenberg, No. 341), and
 +
of Criodrilus (Hatschek, No. 339), is more unequal and more irregular than
 +
that of Lumbricus agricola, and there is an invagination which is intermediate between the embolic and epibolic types.
 +
 +
The segmentation of Lumbricus trapezoides is especially remarkable. It
 +
is strangely irregular and at one period the segmentation cavity communicates by a pore with the exterior. Before the completion of the gastrula
 +
stage the ovum becomes partially divided into two halves, each of which
 +
gives rise to a complete embryo. The two embryos are at first united
 +
by an epiblast cord which connects their necks (fig. 141 A), but this cord is
 +
very early ruptured, and the two embryos then become quite independent.
 +
Some of the peculiarities of the segmentation may no doubt be explained
 +
by this remarkable embryonic fission.
 +
 +
The gastrula opening in both Lumbricus trapezoides and Criodrilus is
 +
placed on the ventral surface, and eventually narrows to form the mouth
 +
or possibly (Criodrilus) closes at the position of the mouth. In Lumbricus
 +
trapezoides the oral opening is at first lined by hypoblast, and in Criodrilus
 +
is bounded anteriorly by three large peculiar epiblast cells, which are
 +
 +
1 According to Willemoes-Suhm, Terebellides stroemii is also characterised by a
 +
regular segmentation,
 +
 +
3, II, 21
 +
 +
 +
 +
 +
322 FORMATION OF THE LAYERS.
 +
 +
believed by Hatschek to assist in absorbing the albuminous fluid in which
 +
the eggs are suspended. These large cells are eventually covered by the
 +
normal epiblast cells and subsequently disappear. In both these types
 +
the hypoblast cells undergo, during their invagination, peculiar changes
 +
connected with their nutritive function.
 +
 +
In Euaxes (Kowalevsky) the segmentation is far more unequal than in
 +
the other types ; a typical epibolic invagination takes place (fig. 140), and
 +
the blastopore closes completely along the ventral surface.
 +
 +
In all the oligochaetous types, with the exception of Euaxes,
 +
where the blastopore closes completely, the blastopore becomes,
 +
or coincides with the mouth. In
 +
Serpula it is stated (Stossich),
 +
as we have seen, to coincide
 +
with the anus : a statement
 +
which receives confirmation
 +
from the similar statements of
 +
Willemoes-Suhm (No. 358). It
 +
is necessary either to suppose ^
 +
 +
a mistake on the part of Stossirh or that we have in Chaeto- FlG - l *' TRANSV EKSE SECTION
 +
 +
S1Cn > 01 THROUGH THE OVUM OF EUAXES
 +
 +
pods a case like that of Gas- DURING AN EARLY STAGE OF DEVELOP, . 1-1 f. 1-1 MENT. (After Kowalevsky.)
 +
 +
teropods in which a slit-like ^ epiblast; ms. mesoblastic band;
 +
blastopore originally extending h- hypoblast.
 +
along the ventral surface may in some forms become reduced
 +
to a pore at the oral, or in other forms at the anal extremity.
 +
 +
So far only two germinal layers the epiblast and the hypoblast have been spoken of. Before the invagination of the
 +
hypoblast is completed the mesoblast makes its appearance in
 +
the form of two bands or streaks, extending longitudinally for
 +
the whole length of the embryo. These are usually spoken
 +
of as germinal streaks, but to avoid the ambiguity of this term
 +
they will be spoken of as mesoblastic bands.
 +
 +
Their origin and growth has been most fully studied by
 +
Kleinenberg (No. 341) in Lum. trapezoides. They commence
 +
in this species shortly before the gastrula stage as two large
 +
cells on the surface of the blastoderm, which may be called
 +
mesoblasts. These cells lie one on each side of the median
 +
line at the hind end of the embryo. They soon travel inwards
 +
and become covered by the epiblast (fig. 141 A, m'\ while on
 +
their inner and anterior side a row of small cells appears (ms).
 +
 +
 +
 +
CH^TOPODA.
 +
 +
 +
 +
323
 +
 +
 +
 +
These rows of cells form the commencement of the mesoblastic
 +
bands, and in the succeeding stages they extend one on each
 +
side of the body (fig. 141 B, ms) till they reach the sides of the
 +
mouth. Their forward growth takes place mainly at the
 +
expense of the superjacent epiblast cells, but the two mesoblasts
 +
 +
 +
 +
 +
FIG. 141. THREE SECTIONS ILLUSTRATING THE DEVELOPMENT OF LUMBRICUS
 +
 +
TRAPEZOIDES. (After Kleinenberg.)
 +
ms. mesoblastic band ; m' . mesoblast ; al. archenteron ; pp. body cavity.
 +
 +
A. Horizontal and longitudinal section of an embryo which is dividing into two
 +
embryos at the gastrula stage. It shews the mesoblasts and the mesoblastic bands
 +
proceeding from them.
 +
 +
B. Transverse section shewing the two widely-separated mesoblastic bands.
 +
 +
C. Transverse section at a later stage shewing the mesoblastic bands which have
 +
approached the ventral line and developed a body cavity^/.
 +
 +
at their hinder extremities probably assist in their growth.
 +
Each mesoblastic band is at first composed of only a single row
 +
of cells, but soon becomes thicker, first of all in front, and
 +
becomes composed of two, three or more rows of cells abreast.
 +
From the above it is clear that the mesoblastic bands have, in
 +
L. trapezoides at any rate, in a large measure an epiblastic
 +
origin.
 +
 +
At first the two bands end in front at the sides of the mouth,
 +
but subsequently their front ends grow dorsalwards at the
 +
 +
21 2
 +
 +
 +
 +
324 FORMATION OF THE LAYERS.
 +
 +
expense of the adjoining epiblast cells, and meet above the
 +
mouth, forming in this way a mesoblastic dorsal commissure.
 +
 +
The mesoblastic bands soon travel from the lateral position,
 +
which they at first occupy, towards the ventral surface. They
 +
do not however meet ventrally for some time, but form two
 +
bands, one on each side of the median ventral line (fig.
 +
141 C).
 +
 +
The usual accounts of the origin and growth of the bands differ somewhat from the above. By Kowalevsky (No. 342) and Hatschek (No. 339)
 +
they are believed to increase in Lumbricus rubellus and Criodrilus entirely
 +
at the expense of the mesoblasts. Kowalevsky moreover holds that in L.
 +
rubellus the original mesoblasts spring from the hypoblast. In some forms,
 +
e.g. Lumbricus agricola, the mesoblasts are not present.
 +
 +
In Euaxes the origin of the mesoblast bands is somewhat interesting
 +
as illustrating the relation of the Chaetopod mesoblastic bands to the
 +
mesoblast of other forms. To render intelligible the origin of the mesoblast
 +
in this form, it is necessary to say a few words about the segmentation.
 +
 +
By a somewhat abnormal process of segmentation the ovum divides
 +
into four spheres, of which one is larger than the others, and occupies
 +
a position corresponding with the future hind end of the embryo. The
 +
three smaller spheres give rise on their dorsal side by a kind of budding
 +
to small cells, which become the epiblast ; and the epiblast is also partly
 +
formed from the hinder large cell in that this cell produces by budding
 +
a small cell, which again divides into two. The anterior of the two
 +
cells so formed divides still further and becomes incorporated in the
 +
epiblast ; the posterior only divides into two which form the two mesoblasts.
 +
The remainder of the mesoblast is formed by further division of the three
 +
smaller of the primitive large spheres, and at first forms a continuous
 +
layer between the dorsal cap of epiblast and the four largest cells which,
 +
after giving rise to the epiblast and mesoblast, constitute the hypoblast.
 +
As the epiblast spreads over the hypoblast the mesoblastic sheet gives way
 +
in the middle, and the mesoblast remains as a ridge of cells at the edge of
 +
the epiblastic cup. It forms in fact a thickening of the lips of the blastopore. Behind the thickening is completed by the two mesoblasts. The
 +
appearance of the mesoblast in section is shewn in fig. 140. As the
 +
epiblast accompanied by the mesoblast grows round the hypoblast, the
 +
blastopore assumes an oval form, and the mesoblast appears as two bands
 +
forming the sides of the oval. The epiblast travels over the hypoblast
 +
more rapidly than the mesoblast, so that when the blastopore becomes
 +
closed ventrally the mesoblastic bands are still some little way apart on
 +
the ventral side.
 +
 +
In Euaxes the mesoblast originates in a manner which is very
 +
similar to that in some of the Gasteropoda, e.g. Nassa, vide p. 234,
 +
and Vermes, e.g. Bonellia, etc. As mentioned in the chapter on the
 +
 +
 +
 +
CH^TOPODA. 325
 +
 +
 +
 +
Mollusca the origin of the mesoblast in Planorbis, p. 227, is very similar to
 +
that in Lumbricus.
 +
 +
Hatschek has shewn that in Polygordius the mesoblast arises in fundamentally the same way as in the Oligochaeta.
 +
 +
Besides the mesoblast which arises from the mesoblastic bands, there
 +
is evidence of the existence of further mesoblast in the larvae of many
 +
Polychaeta in the form of muscular fibres which traverse the space between
 +
the body wall and the wall of the enteric cavity prior to the formation
 +
of the permanent body cavity. These fibres have already been described
 +
in the embryo of Serpula, and are probably represented by stellate cells
 +
in the cephalic region (pras-oral lobe) of the Oligochaeta. These cells are
 +
probably of the same nature as the amoeboid cells in the larvae of Echinodermata, some Mollusca and other types.
 +
 +
The Larval form.
 +
 +
True larval forms are not found in the Oligochaeta where the
 +
development is abbreviated. They occur however in the majority of the marine Polychseta.
 +
 +
They present a great variety of characters with variously
 +
arranged ciliated bands. Most of these forms can be more or
 +
less satisfactorily derived from a larval form, like that of Serpula
 +
(fig. 139 B) or Polygordius (fig. 142); and the constant recurrence of this form amongst the Chsetopoda, combined with the
 +
fact that it presents many points of resemblance to the larval
 +
forms of many Rotifers, Molluscs, and Gephyreans, seems to
 +
point to its being a primitive ancestral form for all these
 +
groups.
 +
 +
The important characters of this larval form are (i) the
 +
division of the body into a large prae-oral lobe and a relatively
 +
small post-oral region containing the greater part of the alimentary tract ; (2) the presence of a curved alimentary canal
 +
divided into stomodaeum (oesophagus), stomach and intestine,
 +
and opening by a ventrally placed mouth, and an anus near the
 +
hind end of the body. To these may be added the frequent
 +
presence of (i) a ganglion at the apex of the prae-oral lobe,
 +
(2) a large cavity between the wall of the gut and the skin,
 +
which is the remnant of the segmentation cavity, and is usually
 +
traversed by muscular strands, of which one connecting the apex
 +
of the prae-oral lobe and the stomach or oesophagus is very
 +
commonly present (fig. 142).
 +
 +
The arrangement of the ciliated bands presents great varia
 +
 +
 +
326
 +
 +
 +
 +
THE LARVAL FORM.
 +
 +
 +
 +
me.p
 +
 +
 +
 +
nph
 +
 +
 +
 +
tions, though in some instances it is constant through large
 +
groups. In Chaetopods there is a widely distributed prae-oral
 +
ciliated band, which is similarly placed to the ring constantly
 +
found in the larvae of Molluscs, Rotifers, etc. In many of these
 +
forms the band is practically double, the opening of the mouth
 +
being placed between its two component rings (vide fig. 142).
 +
The best introduction to the study of the Chaetopod larval forms
 +
will be the history of the changes of a typical larval form in
 +
becoming converted into the adult.
 +
 +
For this purpose no better form can be selected than the interesting
 +
larva of Polygordius (vide Agassiz, No. 332,
 +
Schneider, No. 352, and Hatschek, No. 339),
 +
which was first discovered by Lovdn, and
 +
believed by him to be the larva of an ordinary
 +
Chaetopod. Its true nature was determined
 +
by Schneider.
 +
 +
At a very young stage the larva has the
 +
form (fig. 142) of a flattened sphere, with a
 +
small conical knob at the posterior extremity.
 +
 +
At the equator are situated two parallel
 +
ciliated bands 1 , between which lies the ventrally placed mouth (ni). The more conspicuous ciliated band is formed of a double row
 +
of cilia, and is situated in front of the mouth. The thinner ciliated band
 +
behind the mouth appears to be absent in the American species.
 +
 +
The mouth leads into an oesophagus, and this into a globular stomach
 +
(<?/), which is continuous with a rectum terminating
 +
by an anus (an) placed at the hind end of the
 +
posterior conical knob. The whole alimentary
 +
tract is ciliated. In the American form of larva
 +
there is a ring of cilia round the anus, which is
 +
developed at a somewhat later stage in the form
 +
observed by Hatschek.
 +
 +
The position of the ciliated bands and the
 +
alimentary tract enables us to divide the embryo
 +
into three regions : a prae-oral region bounded by
 +
the anterior ciliated band, a gastric region in
 +
which the embryonic stomach is situated, and an
 +
abdominal region formed of the posterior conical
 +
 +
 +
 +
 +
FlG. 142. POLYGORDIUS
 +
 +
LARVA. (After Hatschek.)
 +
 +
m. mouth; sg. supra-cesophageal ganglion ; nph. nephridion ; me.p. mesoblastic band ;
 +
an. anus ; ol. stomach.
 +
 +
 +
 +
 +
FIG. 143. POLYGORDIUS LARVA . ( From Alex .
 +
Agassiz.)
 +
 +
 +
 +
1 These two rings are at first (Hatschek) not quite closed dorsally, calling to mind
 +
the early condition of the Echinoderm larva with a prae-oral and post-oral ciliated
 +
 +
 +
 +
CH^ETOPODA.
 +
 +
 +
 +
327
 +
 +
 +
 +
 +
FlG. 144. POLYGORDIUS LARVA. (From
 +
Alex. Agassiz.)
 +
 +
 +
 +
portion, which by its subsequent elongation gives rise to the whole
 +
segmented portion of the future Polygordius.
 +
 +
At the front end of the prae-oral lobe is situated the early formed supracesophageal ganglion (sg) (first noticed by Agassiz) in connection with
 +
which is a pair of eyes, and a ramified system of nerves. The ganglion is
 +
marked externally by a crown of cilia.
 +
 +
The larval epidermis bears a delicate cuticula, and is separated by a
 +
considerable interval from the walls of the alimentary tract. The space
 +
between the two represents a provisional body cavity, which is eventually
 +
replaced by the permanent body cavity formed between the two layers
 +
of the mesoblast. It is doubtful when the replacement takes place in the head. It probably does so
 +
very early. The mesoblast is present in the usual
 +
form of two bands (me.p] (germinal streaks), which
 +
are anteriorly continued into two muscular bands
 +
which pass through the embryonic body cavity to
 +
the front end of the prae-oral lobe. Another pair of
 +
contractile bands passes from the same region of the
 +
prse-oral lobe to the oesophagus.
 +
 +
There is no trace of the ventral nerve cord. The
 +
most remarkable organ of the larva is a paired excretory organ (npti) discovered by Hatschek. This is a
 +
ciliated canal with at first one and subsequently
 +
several funnel-shaped openings into the body cavity in front and an
 +
external opening behind. It is situated immediately anterior to the lateral
 +
band of mesoblast, and is parallel with, and dorsal
 +
to, the contractile band which passes off from
 +
this. It occupies therefore a position in front
 +
of the segmented region of the adult Polygordius.
 +
 +
The changes by which this peculiar larval form
 +
reaches the adult condition will be easily gathered
 +
from an inspection of figs. 143 148. They consist essentially in the elongation of what has been
 +
termed the abdominal region of the body, and the
 +
appearance of a segmentation in the mesoblast ;
 +
the segments being formed from before backwards,
 +
and each fresh segment being interpolated between
 +
the anus-bearing end of the body and the last
 +
segment.
 +
 +
As the hind portion of the body becomes
 +
elongated the stomach extends into it, and gives rise to the mesenteron
 +
of the adult (figs. 143, 144, and 145). For a long time the anterior spherical
 +
dilated portion of the larva remains very large, consisting of a prae-oral
 +
lobe and a post-oral section. The two together may be regarded as constituting the head.
 +
 +
At a comparatively late stage a pair of tentacles arises from the front
 +
 +
 +
 +
 +
FIG. 145. POLYGORDIUS LARVA. (From
 +
Alex. Agassiz.)
 +
 +
 +
 +
328
 +
 +
 +
 +
THE LARVAL FORM.
 +
 +
 +
 +
end of the prae-oral lobe (fig. 146), and finally the head becomes relatively
 +
reduced as compared with the body, and gives rise to the simple head of
 +
 +
 +
 +
 +
FIG. 146. POLYGORDIUS LARVA. (From Alex.
 +
Agassiz.)
 +
 +
the fully formed worm (fig. 148). The two ciliated bands disappear, the
 +
posterior vanishing first. The ciliated band at the hind end of the body
 +
also atrophies ; and just in front of it the ring of wart-like prominences used
 +
by the adult to attach itself becomes developed.
 +
 +
At the sides of the head there is formed a pair of ciliated pits, also
 +
found by Hatschek in the embryo of
 +
Criodrilus, and characteristic of many
 +
Chaetopod larvae, but persistent in the
 +
adult Polygordius, Saccocirrus, Polyophthalmus, etc. They are perhaps the
 +
same structures as the ciliated pits in
 +
Nemertines.
 +
 +
During the external changes above
 +
described, by which the adult form of
 +
Polygordius is reached, a series of internal changes also takes place which are
 +
for the most part the same as in other
 +
Chaetopoda ; and do not require a detailed
 +
description. The nervous 1 and muscular
 +
systems have precisely the normal development. The division of the mesoblast into somites is not externally indicated. The organs most worthy of
 +
notice are the excretory organs.
 +
 +
 +
 +
 +
FIG. 147. POLYGORDIUS LARVA.
 +
(From Alex. Agassiz.)
 +
 +
 +
 +
The essential points in the above development of Polygordius are (i) the gradual elongation and corresponding segmentation of the post-cephalic part of the body ; and (2) the relative
 +
reduction in size of the prae-oral lobe and its conversion together with the oral region into the head ; (3) the atrophy of
 +
the ciliated bands. The conversion of the larva into the adult
 +
takes place in fact by the intercalation of a segmented region
 +
 +
 +
 +
1 The structure of the ventral cord in the adult requires further elucidation.
 +
 +
 +
 +
CH^TOPODA.
 +
 +
 +
 +
329
 +
 +
 +
 +
between a large mouth-bearing portion of the primitive body
 +
and a small anus-bearing portion 1 .
 +
 +
The general mode of development of Chsetopod larvae is
 +
similar to the above except in details, which are however no
 +
doubt often of great importance. The history of the larvae may
 +
 +
 +
 +
 +
FIG. 148. POLYGORDIUS LARVA. (From Alex. Agassiz.)
 +
 +
be conveniently treated under three heads, (i) The form of
 +
the primitive unsegmented larva; (2) the arrangement of the
 +
cilia on the unsegmented larva, and on the larva at later stages ;
 +
(3) the character of the metamorphosis and the development of
 +
the permanent external organs.
 +
 +
A larva similar to the Polygordius larva with a greatly
 +
developed prae-oral lobe is widely distributed amongst the
 +
Annelids.
 +
 +
 +
 +
An almost identical form is that of Nepthys
 +
scolopendroides (Claparede and Metschnikoff, No.
 +
336) ; that of Phyllodoce (fig. 149) is also very
 +
similar, and that of Saccocirrus (Metsch. and Clap.
 +
No. 336, PL XIII. fig. i), a very primitive form most
 +
nearly related to Polygordius, clearly belongs to the
 +
same type. Many other larval forms, such as that
 +
of Spio fuliginosus (Metsch. and Clap. No. 336), Terebella, Nerine, etc., also closely approach this form.
 +
 +
 +
 +
 +
FIG. 149. LARVA OF
 +
PHYLLODOCE. (From
 +
Alex. Agassiz.)
 +
 +
 +
 +
Other really similar forms at first sight
 +
appear very different, but this is mainly
 +
owing to the fact that their prae-oral lobe never attains a
 +
considerable development. Its smallness, though obviously of
 +
no deep morphological significance, at once produces a very
 +
different appearance in a larva.
 +
 +
 +
 +
1 For Semper's view as to the intercalation of segments in the cephalic region,
 +
vide note on p. 333.
 +
 +
 +
 +
330
 +
 +
 +
 +
THE LARVAL FORM.
 +
 +
 +
 +
A good example of a larval form with a small prae-oral lobe is afforded by
 +
Capitella, which is figured by Clap, and Metsch. (No. 336, PI. xvn. fig. 2).
 +
The imperfect development of the prae-oral lobe is also generally characteristic of the Oligochasta. The persistence of a relatively large pras-oral lobe
 +
for so long a time as in Polygordius is very unusual.
 +
 +
The arrangement of the cilia in Chaetopod larvae has been
 +
employed as a means of classifying them. Although a classification so framed has no morphological value, yet the terms
 +
themselves which have been invented are convenient. The
 +
terms most usually employed are Atrochae, Monotrochae,
 +
Telotrochae, Polytrochae, Mesotrochae. The polytrochae
 +
may again be subdivided into Polytrochae proper, Nototrochae,
 +
Gasterotrochae, and Amphitrochae.
 +
 +
The atrochae contain forms (fig. 139) in which the larva is at
 +
first coated by an uniform covering of cilia, which, though it
 +
may subsequently disappear from certain areas, does not break
 +
up into a series of definite bands.
 +
 +
The monotrochae or cephalotrochae are larvae in which only a
 +
single prae-oral ring is developed (fig. 150 B).
 +
 +
In the telotrochae there is
 +
present a prae-oral and a postoral, i.e. peri-anal ring (fig. 150
 +
A) ; the latter sometimes having the form of a peri-anal
 +
patch.
 +
 +
The polytrochae are segmented larvae with perfect or
 +
imperfect rings of cilia on the
 +
segments of the body usually
 +
one ring to each segment
 +
between the two characteristic
 +
 +
 +
 +
 +
FIG. 150. Two CH>ETOPOD LARVAE.
 +
 +
(From Gegenbaur.)
 +
 +
o. mouth ; i. intestine ; a. anus ;
 +
v. pne-oral ciliated band ; w. peri-anal
 +
ciliated band.
 +
 +
 +
 +
telotrochal rings. When these
 +
 +
rings are complete the larvae
 +
 +
are polytrochae proper, when they are only half rings they are
 +
 +
either nototrochae or gasterotrochae. Sometimes there are both
 +
 +
dorsal and ventral half rings which do not however correspond,
 +
 +
such forms constitute the amphitrochae.
 +
 +
In the mesotrochae one or two rings are present in the middle
 +
of the body, and the characteristic telotrochal rings are absent.
 +
 +
 +
 +
CKLETOPODA. 331
 +
 +
 +
 +
Larvae do not necessarily continue to belong to the same group
 +
at all ages. A larva may commence as a monotrochal form and
 +
then become telotrochal and from this pass into a polytrochal
 +
condition, etc.
 +
 +
The atrochal forms are to be regarded as larvae which never
 +
pass beyond the primitive stage of uniform ciliation, which in
 +
other instances may precede that of definite rings. They usually
 +
lose their cilia early, as in the cases of Serpula and other larvae
 +
described below.
 +
 +
The atrochal larvae are not common. The following history of an
 +
Eunicidan larva (probably Lumbriconereis) from Claparede and Metschnikoff (No. 336) will illustrate their general history.
 +
 +
In the earliest stage noticed the larva has a spherical form, the prae-oral
 +
lobe not being very well marked. In the interior is a globular digestive
 +
tract. The cilia form a broad central band leaving free a narrow space at
 +
the apex of the prae-oral lobe, and also a circumanal space. At the apex of
 +
the pras-oral lobe is placed a bunch of long cilia, and a patch of cilia also
 +
marks out the anal area.
 +
 +
As the larva grows older it becomes elongated, and the anterior bunch of
 +
cilia is absorbed. The alimentary canal divides itself into pharynx and
 +
intestine. The former opens (?) by the mouth in the middle of the central
 +
band of cilia, the latter in the anal patch. The setae indicating the segmentation are formed successively in the posterior ring-like area free from cilia.
 +
The cilia disappear after the formation of two segments.
 +
 +
In Lumbricus, the embryo of which ought perhaps to be grouped with
 +
the atrochae, the cilia (Kleinenberg) cover a ventral tract of epiblast between
 +
the two mesoblastic cords, and are continued anteriorly to form a circle
 +
round the mouth.
 +
 +
The monotrochal larvae are provided only with the important
 +
prae-oral ciliated ring before mentioned. In the majority of
 +
cases they are transitional forms destined very shortly to become
 +
telotrochal, and in such instances they usually have a more or
 +
less spherical body which is nearly divided into two equal halves
 +
by a ciliated ring. In some few instances, such as Polynoe,
 +
Dasychone, etc., the monotrochal characters are not lost till the
 +
larval cilia are exuviated.
 +
 +
The telotrochal forms (of which examples are shewn in figs.
 +
144, 150, etc.) may (i) start as monotrochal; or (2) from the
 +
first have a telotrochal character ; or (3) be derived from atrochal
 +
forms. The last mode of origin probably represents the ancestral one.
 +
 +
 +
 +
332 LARVAL FORMS.
 +
 +
 +
 +
Their mode of development is well illustrated by the case of Terebella
 +
nebulosa (vide Milne-Edwards, No. 347). The embryo is at first a nearly
 +
spherical ciliated mass. One end slightly elongates and becomes free from
 +
cilia, and, acquiring dorsally two eye-spots, constitutes a prse-oral lobe.
 +
The elongation continues at the opposite end, and near this is formed a
 +
narrow area free from cilia. The larva now has the same characters as the
 +
atrochal Eunicidan larva described above. It consists of a non-ciliated
 +
prae-oral lobe, followed by a wide ciliated band, behind which is a ring-like
 +
area free from cilia ; and behind this again a peri-anal patch of cilia. The
 +
ring-like area free from cilia is, as in the Eunicidan larva, the region which
 +
becomes segmented. It soon becomes longer, and is then divided into two
 +
segments ; a third and fourth etc. non-ciliated segment becomes successively interposed immediately in front of the peri-anal patch ; and, after
 +
a certain number of segments have become formed, there appear on some of
 +
the hinder of them short tubercles, provided with single setae (the notopodia),
 +
which are formed from before backwards, like the segments.
 +
 +
The mouth, anus, and intestine become in the meantime clearly visible.
 +
The mouth is on the posterior side of the ciliated band, and the anus in the
 +
centre of the peri-anal patch.
 +
 +
The ciliated band in front now becomes contracted and provided with
 +
long cilia. It passes below completely in front of the mouth, and constitutes,
 +
in fact, a well-marked pras-oral ring, while the cilia behind constitute an
 +
equally marked peri-anal ring. The larva has in fact now acquired all the
 +
characters of a true telotrochal form.
 +
 +
Only a comparatively small number of Chsetopod larvae
 +
remain permanently telotrochal. Of these Terebella nebulosa,
 +
already cited (though not Terebella conchilega), is one ; Polygordius, Saccocirrus and Capitella are other examples of the
 +
same, though in the latter form the whole ventral surface
 +
becomes ciliated.
 +
 +
The majority of the originally telotrochal forms become
 +
polytrochal.
 +
 +
In most cases the ciliated rings or half rings of the polytrochal forms are placed at equal distances, one for each segment.
 +
They are especially prominent in surface-swimming larvae, and
 +
are in rare cases preserved in the adult. In some instances
 +
(e.g. Nerine and Spio) the ventral half rings, instead of being
 +
segmentally arranged, are somewhat irregularly distributed
 +
amongst the segments, so that there does not seem to be a
 +
necessary correspondence between the ciliated rings and the
 +
segments. This is further shewn by the fact that the ciliated
 +
rings are not precursors of the true segmentation, but are
 +
 +
 +
 +
CH^TOPODA. 333
 +
 +
 +
 +
developed after the establishment of the segments, and thus
 +
seem rather to be secondarily adapted to the segments than
 +
primarily indicative of them.
 +
 +
In most Polytrochae the rings are incomplete, so that they
 +
fall under the category of Nototrochae or Gasterotrochae.
 +
 +
The larva of Odontosyllis is an example of the former, and that of
 +
Magelona of the latter. The larvae of Nerine and Spio, already quoted as
 +
examples of an unsegmented arrangement of the ventral ciliated half rings,
 +
are both amphitrochal forms.
 +
 +
As an example of a polytrochal form with complete ciliated rings Ophryotrocha puerilis may be cited. This form, discovered by Claparede and
 +
Metschnikoff, develops a complete ciliated ring on each segment : and the
 +
prae-oral ring, though at first single, becomes at a later period divided into
 +
two. This form is further exceptional in that the ciliated rings are persistent
 +
in the adult.
 +
 +
The unimportance of the character of the rings in the polytrochal forms
 +
is shewn by such facts as the absence of these rings in Terebella nebulosa
 +
and the presence of dorsal half rings in Terebella conchilega.
 +
 +
The mesotrochal forms are the rarest of Chaetopod larvae,
 +
and would seem to be confined to the Chaetopteridae.
 +
 +
Their most striking character is the presence of one or two complete
 +
ciliated rings which girth the body between the mouth and anus. The
 +
whole body is further covered with short cilia. The anus has a distinct
 +
dorsal situation, while on its ventral side there projects backwards a peculiar
 +
papilla.
 +
 +
The total absence of the typical prae-oral and of the peri-anal
 +
bands separates the mesotrochal larvae very sharply from all the
 +
previous types.
 +
 +
A characteristic of many Chaetopod larvae is the presence of
 +
a bunch of cilia or a single flagellum at the apex of the prae-oral
 +
lobe. The presence of such a structure is characteristic of the
 +
larval forms of many other groups, Turbellarians, Nemertines,
 +
Molluscs, etc.
 +
 +
In the preceding section the mode of multiplication of the
 +
segments has already been sufficiently described 1 .
 +
 +
1 It has been insisted by Semper (No. 355) that certain of the anterior segments,
 +
belonging to what he regards as the head region in opposition to the trunk, become
 +
interpolated between the trunk and the head. The general evidence, founded on observations of budding, which he brings forward, cannot be discussed here. But the
 +
special instance which he cites (founded on Milne-Edwards's (No. 347) observations)
 +
 +
 +
 +
334
 +
 +
 +
 +
LARVAL FORMS.
 +
 +
 +
 +
 +
FIG. 151. LARVA OF
 +
PHYLLODOCE FROM THE
 +
 +
VENTRAL SIDE. (From
 +
 +
Alex. Agassiz.)
 +
 +
 +
 +
Apart from the formation of the segments the larval metamorphosis consists in the atrophy of the
 +
provisional ciliated rings and other provisional organs, and in the acquirement of
 +
the organs of the adult.
 +
 +
The great variations in the nature of the
 +
Chaetopod appendages render it impossible
 +
to treat this part of the developmental
 +
history of the Chaetopoda in a systematic
 +
way.
 +
 +
The mode of development of the appendages is not constant, so that it is difficult to
 +
draw conclusions as to the primitive form
 +
from which the existing types of appendages
 +
are derived.
 +
 +
In a large number of cases the primitive rudiments of the
 +
feet exhibit no indication of a division into notopodium and
 +
neuropodium ; while in other instances (e.g. Terebella and Nerine,
 +
fig. 152) the notopodium is first
 +
developed, and subsequently the
 +
neuropodium quite independently.
 +
 +
In many cases the setae appear
 +
before there are any other visible
 +
rudiments of the feet (e.g. Lumbriconereis) ; while in other cases the
 +
reverse holds good. The gills arc
 +
usually the last parts to appear.
 +
 +
Not only does the mode of
 +
development of the feet differ
 +
greatly in different types, but also the period. The appearance
 +
of setae may afford the first external indication of segmentation,
 +
or the rudiments of the feet may not appear till a large number
 +
of segments are definitely established.
 +
 +
A very considerable number of Chaetopod larvae are provided
 +
with very long provisional setae (figs. 152 and 153). These setae
 +
 +
of the interpolation of the head segments, bearing the gills, in Terebella appears to
 +
me quite unjustified from Milne-Edwards's own statements ; and is clearly shewn to
 +
be unfounded by the careful observations of Claparede on Ter. conchilega, where the
 +
segments in question are demonstrated to be present from the first.
 +
 +
 +
 +
 +
FIG. 152. LARVA OF NERINE,
 +
 +
WITH PROVISIONAL SET>E. (From
 +
 +
Alex. Agassiz.)
 +
 +
 +
 +
CH^iTOPODA.
 +
 +
 +
 +
335
 +
 +
 +
 +
are usually placed at the sides of the anterior part of the body,
 +
immediately behind the head, and also sometimes on the
 +
posterior parts of the body. In some instances (e.g. fig. 153)
 +
 +
 +
 +
 +
FIG. 153. EMRRYO CH^ETOPOU WITH PROVISIONAL SEIVE. (From Agassiz.)
 +
 +
they form the only appendages of the trunk. Alex. Agassiz .has
 +
pointed out that setae of this kind, though not found in existing
 +
Chaetopods, are characteristic of the fossil forms. Setae of this
 +
kind are found in chaetopod-like larvae of some Brachiopods
 +
(Argiope, fig. 136).
 +
 +
It is tempting to suppose that the long provisional bristles
 +
springing from the oral region are the setiform appendages
 +
handed down from the unsegmented ancestors of the existing
 +
Chaetopod forms. Claparede has divided Chaetopod larvae into
 +
two great groups of Metachaetae and Perennichaetae, according as
 +
they possess or are without provisional setae.
 +
 +
With reference to the head and its appendages it has already
 +
been stated that the head is primarily formed of the prae-oral
 +
lobe and of the peristomial region.
 +
 +
The embryological facts are opposed to the view that the
 +
prae-oral region either represents a segment or is composed of
 +
segments equivalent to those of the trunk. The embryonic
 +
peristomial region may, on the other hand, be regarded as in a
 +
certain sense the first segment. Its exact relations to the
 +
succeeding segments become frequently more or less modified in
 +
the adult. The prae-oral region is in most larvae bounded
 +
behind by the ciliated ring already described. On the dorsal
 +
part of the prae-oral lobe in front of this ring are placed the
 +
eyes, and from it there may spring a variable number of
 +
processes which form antennae or cephalic tentacles. The
 +
number and position of these latter are very variable. They
 +
appear as simple processes, sometimes arising in pairs, and at
 +
 +
 +
 +
336 LARVAL FORMS.
 +
 +
 +
 +
other times alternating on the two sides. There is frequently a
 +
median unpaired tentacle.
 +
 +
The development of the median tentacle in Terebella, where there is in
 +
the adult a great number of similar tentacles, is sufficiently remarkable to
 +
deserve special notice ; vide Milne-Edwards, Claparede, etc. It arises long
 +
before any of the other tentacles as a single anterior prolongation of the
 +
prae-oral lobe containing a parenchymatous cavity, which communicates
 +
freely with the general perivisceral cavity. It soon becomes partially constricted off at its base from the procephalic lobe, but continues to grow
 +
till it becomes fully half as long as the remainder of the body. A very
 +
characteristic figure of the larva at this stage is given by Claparede and
 +
Metschnikoff, PI. XVII., Fig. I E. It now strikingly resembles the larval
 +
proboscis of Balanoglossus, and it is not easy to avoid the conclusion that
 +
they are homologous structures.
 +
 +
Another peculiar cephalic structure which deserves notice is the gill
 +
apparatus of the Serpulidae.
 +
 +
In Dasychone (Sabella) the gill apparatus arises (Claparede and
 +
Metschnikoff, No. 336) as a pair of membranous wing-like organs on the
 +
dorsal side of the prae-oral lobe immediately in front of the ciliated ring.
 +
Each subsequently becomes divided into
 +
two rays, and new rays then begin to sprout
 +
on the ventral side of the two pairs already
 +
present. A cartilaginous axis soon becomes
 +
formed in these rays, and after this is
 +
formed fresh rays sprout irregularly from
 +
the cartilaginous skeleton.
 +
 +
In Spirorbis spirillum as observed by
 +
Alex. Agassiz, the right gill-tentacle (fig.
 +
154, /) first appears, and then the left, and
 +
subsequently the odd opercular tentacle
 +
which covers the right original tentacle. FIG. 154. LARVA OF SPIROR
 +
The third and fourth tentacles are formed BIS. (From Alex. Agassiz.)
 +
successively on the two sides, and rapidly The first odd tentacle (t) is shewn
 +
 +
become branched in the succeeding stages. on J\?e ht , side
 +
Behind the prse-oral ciliated ring
 +
 +
With reference to the sense
 +
 +
organs it may be noted that the eyes, or at any rate the cephalic
 +
pigment spots, are generally more numerous in the embryo than
 +
in the adult, and that they are usually present in the larvae of
 +
the Sedentaria, though absent in the adults of these forms. The
 +
Sedentaria thus pass through a larval stage in which they
 +
resemble the Errantia.
 +
 +
Paired auditory vesicles of a provisional character have been
 +
found on the ventral side of the body, in the fourth segment
 +
 +
 +
 +
 +
CH^LTOPODA.
 +
 +
 +
 +
337
 +
 +
 +
 +
behind the mouth, in the larva of Terebella conchilega
 +
(Claparede).
 +
 +
Mitraria. A peculiar larval Ch?etopod form known as Mitraria, the
 +
metamorphosis of which was first worked out by Metschnikoff, deserves a
 +
special notice.
 +
 +
This form (fig. 155 A) in spite of its remarkable appearance can easily be
 +
reduced to the normal type of larva.
 +
 +
The mouth (m) and anus (an) (fig. 155 A) are closely approximated, and
 +
situated within a vestibule the edge of which is lined by a simple or lobed
 +
ciliated ring. The shape of the body is somewhat conical. The cavity of
 +
 +
 +
 +
 +
FlG. 155. TWO STAGES IN THE DEVELOPMENT OF MlTRARIA.
 +
 +
(After Metschnikoff.)
 +
 +
m. mouth; an. anus; sg. supra-oesophageal ganglion; br. provisional bristles;
 +
pr.b. prse-oral ciliated band.
 +
 +
the vestibule forms the base of the cone, and at the apex is placed a ciliated
 +
patch (sg). A pair of lobes (br) bear provisional setae. The alimentary
 +
canal is formed of the three normal parts, oesophagus, stomach, and
 +
intestine.
 +
 +
To compare this larva with an ordinary Chsetopod larva one must
 +
suppose that the alimentary canal is abnormally bent, so that the post-oral
 +
ventral surface is reduced to the small space between the mouth and the
 +
anus. The ciliated band surrounding the vestibule is merely the usual
 +
prae-oral band, borne on the very much extended edge of the pras-oral lobe.
 +
The apex of the larva is the front end of the pras-oral lobe with the
 +
usual ciliated patch. The two lobes with provisional bristles are really
 +
dorsal and not posterior.
 +
 +
 +
 +
B. II.
 +
 +
 +
 +
22
 +
 +
 +
 +
338 FORMATION OF ORGANS.
 +
 +
The correctness of the above interpretation is clearly shewn by the
 +
metamorphosis.
 +
 +
The first change consists in the pushing in of a fold of skin, between the
 +
mouth and anus, towards the intestine, which at the same time rapidly
 +
elongates, and forms the axis of a conical projection, which thereupon
 +
becomes segmented and is thereby shewn to be the rudiment of the trunk
 +
(fig. 155 B). On the elongation of the trunk in this way the prae-oral lobe
 +
and its ciliated ring assume an appearance not very dissimilar to the same
 +
structures in Polygordius. At the ciliated apex of the prae-oral lobe a paired
 +
thickening of epiblast gives rise to the supra-cesophageal ganglia (sg). In
 +
the further metamorphosis, the prae-oral lobe and its ciliated ring gradually
 +
become reduced, and finally atrophy in the normal way, while the trunk
 +
elongates and acquires setae. The dorsally situated processes with provisional
 +
setae last for some time, but finally disappear. The young worm then
 +
develops a tube and shews itself as a normal tubicolous Chaetopod.
 +
 +
Formation of Organs.
 +
 +
Except in the case of a few organs our knowledge of the
 +
formation of the organs in the Chaetopoda is derived from
 +
investigations on the Oligochaeta.
 +
 +
The embryo of the Oligochaeta. has a more or less spherical
 +
form, but it soon elongates, and becoming segmented acquires a
 +
distinct vermiform character. The ventral surface is however
 +
for a considerable time markedly convex as compared to the
 +
dorsal.
 +
 +
The ventrally placed mouth is surrounded by a well-marked
 +
lip, and in front of it is placed a small prae-oral lobe.
 +
 +
The epiblast. The epiblast cells at the commencement of
 +
the gastrula stage become much flattened, and on the completion of the invagination form an investment of flattened cells, only thickened in
 +
the neighbourhood of the mesoblastic
 +
bands (fig. 141 B and C). In the Polychaeta at any rate the statements of
 +
several investigators would seem to in
 +
dicate that the cuticle is derived from the FJG i 6 SFCTION
 +
chorion. It is difficult to accept this THROUGH THE HEAD OF A
 +
 +
 +
 +
 +
conclusion, but it deserves further in
 +
VCStigation. Kleinenberg. )
 +
 +
Nervous system. The most im- '-& cephalic ganglion;
 +
 +
, .. cc. cephalic portion of the
 +
 +
portant organ derived from the epiblast body cavity ; x. cesophagus.
 +
 +
 +
 +
CH^TOPODA.
 +
 +
 +
 +
339
 +
 +
 +
 +
 +
FIG. 157. SECTION THROUGH PART
 +
 +
OF THE VENTRAL WALL OF THE
 +
TRUNK OF AN EMBRYO OF LUMBRI
 +
 +
 +
is the nervous system ; the origin of which from this layer was
 +
first established by Kowalevsky (No. 342).
 +
 +
It arises 1 (Kleinenberg, No. 341) from two at first quite
 +
distinct structures, viz. (i) the supra-cesophageal rudiment
 +
and (2) the rudiment of the ventral cord. The former of
 +
these takes its origin as an unpaired dorsal thickening of
 +
the epiblast at the front end of the head (fig. 1 56, e.g.], which
 +
sends two prolongations downwards and backwards to meet the
 +
ventral cord. The latter arises as two independent thickenings
 +
of the epiblast, one on each side
 +
of the ventral furrow (fig. 157, Vg].
 +
These soon unite underneath the
 +
furrow, in the median line, and
 +
after being differentiated into segmentally arranged ganglionic and
 +
interganglionic regions become
 +
separated from the epiblast. Both
 +
the supra-cesophageal and ventral cord become surrounded by a us TRAPEZOIDES. (After Kleinen
 +
berg.)
 +
 +
layer of somatic mesoblast. The Mm longitudinal muscles . S0m so .
 +
 +
junction between the tWO parts of matic mesoblast ; sp. splanchnic me! i . ' i soblast; hy. hypoblast ; Vg. ventral
 +
 +
the central nervous System takes nerve cord ; w. ventral vessel.
 +
 +
place comparatively late.
 +
 +
The mesoblast. It is to Kowalevsky (No. 342) and Kleinenberg (No. 341) that we mainly owe our knowledge of the history
 +
of the mesoblast. The fundamental processes which take place are
 +
(i) the splitting of the mesoblast into splanchnic and somatic
 +
layers with the body cavity between them, (2) the transverse
 +
division of the mesoblast of the trunk into distinct somites.
 +
 +
The former process commences in the cephalic mesoblastic
 +
commissure, where it results in the formation of a pair of cavities
 +
each with a thin somatic and thick splanchnic layer (fig. 156,
 +
cc) ; and thence extends gradually backwards into the trunk
 +
(fig. 141 C, //). In the trunk however the division into somites
 +
precedes the horizontal splitting of the mesoblast. The former
 +
process commences when the mesoblastic bands form widish
 +
columns quite separate from each other. These columns become
 +
 +
 +
 +
1 For further details, vide general chapter on Nervous System.
 +
 +
22 2
 +
 +
 +
 +
340 FORMATION OF ORGANS.
 +
 +
broken up successively from before backwards into somewhat
 +
cubical bodies, in the centre of which a cavity soon appears.
 +
The cavity in each somite is obviously bounded by four walls,
 +
(i) an outer, the somatic, which is the thickest; (2) an inner, the
 +
splanchnic ; and (3, 4) an anterior and posterior. The adjoining
 +
anterior and posterior walls of successive somites unite together
 +
to form the transverse dissepiments of the adult, which subsequently become very thin and are perforated in numerous places,
 +
thus placing in communication the separate compartments
 +
of the body cavity. The somites, though at first confined to a
 +
small area on the ventral side, gradually extend so as to meet
 +
their fellows above and below and form complete rings (fig. 157)
 +
of which the splanchnic layer (sp) attaches itself to the enteric
 +
wall and the somatic (so) to the epiblast. In Polygordius and
 +
probably also Saccocirrus and other forms the cavities of the
 +
somites of the two sides do not coalesce ; and the walls which
 +
separate them constitute dorsal and ventral mesenteries. The
 +
two cavities in the cephalic commissure unite dorsally, but
 +
ventrally open into the first somite of the trunk.
 +
 +
The mesoblastic masses of the head are probably not to be regarded as
 +
forming a pair of somites equivalent to those in the trunk, but as forming
 +
the mesoblastic part of the pras-oral lobe, of which so much has been said in
 +
the preceding pages. Kleinenberg's observations are however of great importance as shewing that the cephalic cavities are simply an anterior part of
 +
the true body cavity.
 +
 +
The splanchnic layer of the head cavity gives rise to the
 +
musculature of the oesophagus.
 +
 +
The somatic layer of the trunk somites becomes converted
 +
into the musculature of the body wall and the external peritoneal layer of body cavity. The first part of the muscular
 +
system to be definitely formed is the ventral band of longitudinal muscles which arises on each side of the nervous system in
 +
contact with the epidermis (fig. 157, m). How the circular
 +
muscles become subsequently formed outside these muscles has
 +
not been made out.
 +
 +
The splanchnic layer of the trunk somites gives rise to the
 +
muscular and connective-tissue wall of the mcscntcron, and also
 +
to the walls of the vascular trunks. The ventral vessel is first
 +
formed (Kowalevsky) as a solid mass of cells which subsequently
 +
 +
 +
 +
CILKTOPODA. 34!
 +
 +
 +
 +
becomes hollowed out. The dorsal vessel in Lumbricus and
 +
Criodrilus is stated by Kowalevsky and Vejdovsky to be formed
 +
by the coalescence of two lateral vessels ; a peculiarity which is
 +
probably to be explained by the late extension of the mesoblast
 +
into the dorsal region.
 +
 +
The layer from which the sacks for the setae and the
 +
segmental organs spring is still doubtful. The sacks for the setae
 +
are believed by Kowalevsky (No. 342) to be epiblastic invaginations, but are stated by Hatschek (No. 339) to be mesoblastic
 +
products. For the development of the segmental organs the
 +
reader is referred to the chapter on the excretory system.
 +
 +
In marine Polychaeta the generative organs are no doubt
 +
mesoblastic products, as they usually spring from the peritoneal
 +
epithelium, especially the parts of it covering the vascular
 +
trunks.
 +
 +
The Alimentary Canal, In Lumbricus the enteric cavity
 +
is formed during the gastrula stage. In Criodrilus the hypoblast
 +
has at first no lumen, but this becomes very soon established.
 +
In Euaxes on the other hand, where there is a true epibolic
 +
gastrula, the mesenteron is at first represented by a solid mass
 +
of yolk (i.e. hypoblastj cells. As the central amongst these
 +
become absorbed a cavity is formed. The protoplasm of the
 +
yolk cells which line this cavity unites into a continuous polynuclear layer containing at intervals masses of yolk. These masses
 +
become gradually absorbed, and the protoplasmic wall of the
 +
mesenteron then breaks up into a cylindrical glandular epithelium
 +
similar to that of the other types.
 +
 +
In Lumbricus and Criodrilus the blastopore remains as the
 +
mouth, but in Euaxes a new mouth or rather stomodaeum is
 +
formed by an epiblastic invagination between the front end of
 +
the two mesoblastic bands. This epiblastic invagination forms
 +
the permanent oesophagus; and in Lumbricus trapezoides and
 +
Criodrilus, where the oral opening is at first lined by hypoblast,
 +
the epiblast soon becomes inflected so as to line the cesophageal
 +
region. The splanchnic mesoblast of the cephalic region subsequently invests the oesophagus, and some of its cells penetrating
 +
between the adjoining epiblast cells give rise to a thick wall for
 +
this part of the alimentary tract ; the original epiblast cells being
 +
reduced to a thin membrane. This mesoblastic wall is sharply
 +
 +
 +
 +
342 ALTERNATIONS OF GENERATIONS.
 +
 +
separated from the muscular wall outside, which is also formed
 +
of splanchnic mesoblast.
 +
 +
The anus is a late formation.
 +
 +
Alternations of generations.
 +
 +
Amongst Chaetopoda a considerable number of forms exhibit
 +
the phenomenon of alternations of generations, which in the
 +
same general way as in the case of the Ccelenterata, is secondarily caused by budding or fission.
 +
 +
The process of fission essentially consists in the division of a
 +
parent form into two zooids by the formation of a zone of fission
 +
between two old rings, which becomes differentiated (i) into an
 +
anal zone in front which forms the anal region of the anterior
 +
zooid, and (2) into a cephalic zone behind which forms the head
 +
and some of the succeeding segments of the posterior zooid.
 +
The anal zone is capable, by growth and successive segmentation, of giving rise to an indefinite number of fresh segments.
 +
 +
In Protula Dysteri, as shewn by Huxley, there is a simple
 +
fission into two in the way described. Sexual reproduction does
 +
not take place at the same time as reproduction by fission,
 +
but both zooids produced are quite similar and multiply
 +
sexually.
 +
 +
In the freshwater forms Nais and Chaetogaster a more or
 +
less similar phenomenon takes place. By a continual process of
 +
growth in the anal zones, and the formation of fresh zones of
 +
fission whenever four or five segments are added in front of an
 +
anal zone, complicated chains of adhering zooids are produced,
 +
each with a small number of segments. As long as the process
 +
of fission continues sexual products are not developed, but eventually the chains break up, the individuals derived from them
 +
cease to go on budding, and, after developing a considerably
 +
greater number of segments than in the asexual state, reproduce
 +
themselves sexually. The forms developed from the ovum then
 +
repeat again the phenomenon of budding, etc., and so the cycle
 +
is continued 1 .
 +
 +
The phenomena so far can hardly be described as cases of
 +
 +
1 Reproduction by budding and formation of the sexual products to some extent
 +
overlap.
 +
 +
 +
 +
CH/ETOPODA. 343
 +
 +
 +
 +
alternation of generations. The process is however in certain
 +
types further differentiated. In Syllis (Quatrefages) fission
 +
takes place, the parent form dividing into two, of which only the
 +
posterior after its detachment develops sexual organs. The
 +
anterior asexual zooid continues to produce fresh sexual zooids
 +
by fission. In Myrianida also, where a chain of zooids is formed,
 +
the sexual elements seem to be confined to the individuals
 +
produced by budding.
 +
 +
The cases of Syllis and Myrianida seem to be genuine
 +
examples of alternations of generations, but a still better
 +
instance is afforded by Autolytus (Krohn, No. 343, and Agassiz,
 +
No. 333).
 +
 +
In Autolytus cornutus the parent stock, produced directly
 +
from the egg, acquires about 40 45 segments, and then gives
 +
rise by fission, with the production of a zone of fission between
 +
about the I3th and I4th rings, to a fresh zooid behind. This
 +
after becoming fully developed into either a male or a female is
 +
detached from the parent stock, from which it very markedly
 +
differs. The males and females are moreover very different from
 +
each other. In the female zooid the eggs are carried into a
 +
kind of pouch where they undergo their development and give
 +
rise to asexual parent stocks. After the young are hatched the
 +
female dies. The asexual stock, after budding off one asexual
 +
zooid, elongates again and buds off a second zooid. It never
 +
develops generative organs.
 +
 +
The life history of some species of the genus Nereis presents certain very
 +
striking peculiarities which have not yet been completely elucidated.
 +
 +
As was first shewn by Malmgren asexual examples of various species of
 +
Nereis may acquire the characters of Heteronereis and become sexually
 +
mature.
 +
 +
The metamorphosis of Nereis Dumerilii has been investigated by
 +
Claparede, who has arrived at certain very remarkable conclusions. He
 +
finds that there are two distinct sexual generations of the Nereis form of
 +
this species, and two distinct sexual generations of the Heteronereis form.
 +
 +
One sexual Nereis, characterized by its small size, is dioecious, the other
 +
discovered by Metschnikoff is hermaphrodite.
 +
 +
Of the Heteronereis sexual forms, both are dioecious, one is small, and
 +
swims on the surface, the other is larger and lives at the bottom.
 +
 +
How these various generations are mutually related has not been made
 +
out ; but Claparede traced the passage of large asexual examples of the
 +
Nereis form into the large Heteronereis form.
 +
 +
 +
 +
344 CH^iTOPODA.
 +
 +
 +
 +
BIBLIOGRAPHY.
 +
 +
(332) Alex. Agassiz. "On the young stages of a few Annelid.^." Annuls
 +
Lyceum Nat. Hist, of New York, Vol. vin. 1866.
 +
 +
(333) Alex. Agassiz. " On the embr}'ology of Autolytus cornutus and alternations of generations, etc." Boston Journal of Nat. History, Vol. VII. 1859 63.
 +
 +
(334) W. Busch. Beobachtnngcn ii. Anat. u. Entwick. einiger wirbel loser Seethiere, 1851.
 +
 +
(335) Ed. Claparede. Beobachlungen ii. Anat. it. Entwick. wirbelloser Thiere
 +
an d. Kiiste von Normandie. Leipzig, 1863.
 +
 +
(336) Ed. Claparede u. E. Metschnikoff. " Beitrage z. Kenntniss iib. Entwicklungsgeschichte d. Chrctopoden." Zeit.f. wt'ss. Zool. Vol. xix. 1869.
 +
 +
(337) E. Grube. Untersuchungen iib. Entwicklung d. Anneliden. Konigsberg,
 +
1844
 +
(338) B. Hatschek. "Beitrage z. Entwick. u. Morphol. d. Anneliden." Sitz.
 +
d. k. Akad. Wiss. Wien, Vol. LXXIV. 1876.
 +
 +
(339) B. Hatschek. " Studien iiber Entwicklungsgeschichte der Anneliden."
 +
Arbeiten aus d. zoologischen Institute d. Universitdt Wien. Von C. Claus. Heft III.
 +
1878.
 +
 +
(340) Th. H. Huxley. "On hermaphrodite and fissiparous species of tubicolar
 +
Annelidae (Protula)." Edinburgh New Phil. Journal, Vol. I. 1855.
 +
 +
(341) N. Kleinenberg. "The development of the earthworm Lumbricus trapezoides." Quart. J. of Micr. Science, Vol. Xix. 1879. Sullo sviluppo del Lumbricus trapezoides. Napoli, 1878.
 +
 +
(342) A. Kowalevsky. " Embryologische Studien an Wurmern u. Arthropoden." Mem. Acad. Petersbourg, Series VII. Vol. XVI. 1871.
 +
 +
(343) A. Krohn. " Ueber die Erscheinungen bei d. Fortpflanzung von Syllis
 +
prolifera u. Autolytus prolifer." Archiv f. Naturgesch. 1852.
 +
 +
(344) R. Leuckart. " Ueb. d. Jugendzustande ein. Anneliden, etc." Archiv
 +
f. Naturgesch. 1855.
 +
 +
(345) S. Loven. " Beobachtungen u. die Metamorphose von Anneliden."
 +
Weigmann's Archiv, 1842.
 +
 +
(346) E. Metschnikoff. " Ueber die Metamorphose einiger Seethiere (Mitraria)." Zeit.f. wiss. Zool. Vol. xxi. 1871.
 +
 +
(347) M. Milne-Edwards. " Recherches zoologiques, etc." Ann. Scie.
 +
Natttr. in. Serie, Vol. in. 1845.
 +
 +
(348) J. M tiller. " Ueb. d. Jugendzustande einiger Seethiere." Monats. d.
 +
k. Akad. Wiss. Berlin, 1851.
 +
 +
(349) Max Muller. "Ueber d. weit. Entwick. von Mesotrocha sexoculata."
 +
Muller's Archiv, 1855.
 +
 +
(350) Quatrefages. " Me"moire s. 1'embryogenie des Annelides." Ann. Scie.
 +
Natur. in. Serie, Vol. x. 1848.
 +
 +
(351) M. Sars. "Zur Entwicklung d. Anneliden." A re hiv f. Naturgeschichte,
 +
Vol. xi. 1845.
 +
 +
(352) A. Schneider. "Ueber Bau u. Entwicklung von Polygordius." Muller's
 +
Archiv, 1868.
 +
 +
(353) A.Schneider. " Entwicklung u. system. Stell. d. Bryozoen u. Gephyreen (Mitraria)." Archiv f. mikr. Anat. Vol. v. 1869.
 +
 +
 +
 +
CHyfcTOPODA. 345
 +
 +
 +
 +
(354) M. Schultze. Ueb. die Entwicklitng von Arenicola piscatorum u. anderer
 +
Kiemenwiirmer . Halle, 1856.
 +
 +
(355) C. Semper. "Die Verwandschaftbeziehungen d. gegliederten Thiere."
 +
Arbeiten a. d. zool.-zoot. Instit. Wurzburg, Vol. in. 1876-7.
 +
 +
(356) C. Semper. " Beitrage z. Biologic d. Oligochseten." Arbeiten a. d. zool.zoot. Instit. Wurzburg, Vol. IV. 1877-8.
 +
 +
(357) M. Stossich. "Beitrage zur Entwicklung d. Chaetopoden." Sitz. d. k.
 +
k. Akad. Whs. Wien, B. LXXVII. 1878.
 +
 +
(358) R. v. Willemoes-Suhm. " Biologische Beobachtungen U. niedrige
 +
Meeresthiere." Zeit. f. wiss. Zool. Bd. xxi. 1871.
 +
 +
 +
 +
CHAPTER XIII.
 +
 +
DISCOPHORA 1 .
 +
 +
THE eggs of the Discophora, each enclosed in a delicate
 +
membrane, are enveloped in a kind of mucous case formed by a
 +
secretion of the integument, which hardens into a capsule or
 +
cocoon. In each cocoon there are a limited number of eggs
 +
surrounded by albumen. The cocoons are attached to waterplants, etc. In Clepsine the embryos leave the cocoon very
 +
soon after they get rid of the egg membrane, but in Nephelis
 +
they remain within the cocoon for a very much longer period
 +
(27 28 days after hatching). The young of Clepsine, after
 +
their liberation, attach themselves to the ventral surface of their
 +
parent.
 +
 +
Our knowledge of the development of the Discophora is in a
 +
very unsatisfactory state ; but sufficient is known to shew that it
 +
has very many points in common with that of the Oligochaeta,
 +
and that the Discophora are therefore closely related to the
 +
Chaetopoda. In Clepsine there is an epibolic gastrula, and
 +
mesoblastic bands like those in Euaxes are also formed. In
 +
Nephelis however the segmentation is very abnormal, and the
 +
formation of the germinal layers cannot easily be reduced to an
 +
invaginate gastrula type, though probably it is modified from
 +
such a type. Mesoblastic bands similar to those in the Oligochaeta occur in this form also.
 +
 +
The embryology of Clepsine, which will serve as type for the
 +
Leeches without jaws (Rhyncobdellidae), has recently been
 +
studied by Whitman (No. 365), and that of Nephelis, which will
 +
 +
1 The Discophora are divided into the following groups.
 +
I. Rhyncobdellidse.
 +
II. Gnathobdellidae.
 +
III. Branchiobdellidffi.
 +
 +
 +
 +
DISCOPHORA.
 +
 +
 +
 +
347
 +
 +
 +
 +
serve as type for the Leeches with jaws (Gnathobdellidae), has
 +
been studied by Butschli (No. 359). The early history of both
 +
types is imperfectly known 1 .
 +
 +
Formation of the layers.
 +
 +
Clepsine. It is necessary to give a full account of the segmentation
 +
of Clepsine, as the formation of the germinal layers would be otherwise
 +
unintelligible.
 +
 +
Segmentation commences with the division of the ovum into two unequal
 +
spheres by a vertical cleavage passing from the animal to the vegetative
 +
pole. By a second vertical cleavage the large segment is divided into two
 +
unequal parts, and the small one into two equal parts. Of the four segments
 +
so produced three are relatively small, and one, placed at the posterior end,
 +
is large. Each of the four segments next gives rise to a small cell at the
 +
animal pole. These small cells form the commencement of the epiblast,
 +
and, according to Whitman, the mouth is eventually placed in their centre.
 +
Such a position for the mouth, at the animal pole, is extremely unusual, and
 +
the statements on this head require further confirmation.
 +
 +
The posterior large segment now divides into two, one of which is dorsal,
 +
and the other and larger ventral. The former I shall call
 +
with Whitman the neuroblast,
 +
and the latter the mesoblast.
 +
The mesoblast very shortly
 +
divides again. During the formation of the neuroblast and
 +
mesoblast additional epiblastic
 +
small cells are added from the
 +
three spheres which give rise
 +
to three of the primitive epiblast cells, which may now be
 +
called the vitelline spheres.
 +
 +
The neuroblast next divides
 +
into ten cells, of which the two
 +
smaller are soon broken up
 +
into epiblastic cells, while the
 +
remaining eight arrange themselves in two groups of four
 +
each, one group on each side
 +
at the posterior border of the epiblastic cap. The two mesoblasts also take up
 +
a position on the right and left sides immediately ventral to the four neuroblasts of each side. The neuroblasts and mesoblasts now commence to
 +
 +
 +
 +
 +
 +
FlG. 158. TWO VIEWS OF THE LARVA OF
 +
 +
CLEPSINE. (After Whitman.)
 +
 +
o. oral extremity ; m mouth ; pr. germinal
 +
streak.
 +
 +
A. This figure shews the blastoderm (shaded)
 +
with a thickened edge formed by the primitive
 +
(i.e. mesoblastic) streaks with the four so-called
 +
neuroblasts posteriorly. The vitelline spheres
 +
are left without shading.
 +
 +
B. represents an embryo in which the blastoderm has enclosed the yolk, and in which the
 +
division into segments has taken place. At the
 +
hind end are shewn the so-called neuroblasts
 +
forming the termination of the germinal streak.
 +
 +
 +
 +
1 Hoffmann's account (No. 36) is so different from that of other observers that
 +
I have been unable to make any use of it.
 +
 +
 +
 +
348 CLEPSINE.
 +
 +
 +
 +
proliferate at their anterior border, and produce on each side a thickened
 +
band of cells underneath the edge of the cap of epiblast cells. Each of these
 +
bands is formed of a superficial quadruple 1 row of neuroblasts budded off
 +
from the four primary neuroblasts, and a deeper row of mesoblasts. The
 +
compound streaks so formed may be called the germinal streaks.
 +
 +
The general appearance of the embryo as seen from the dorsal surface,
 +
after the appearance of the two germinal streaks, may be gathered from
 +
fig. 158 A. The epiblastic cap in this figure is shaded. The epiblastic cap,
 +
accompanied by the germinal streaks, now rapidly extends and encloses the
 +
three vitelline spheres by a process equivalent to that of an ordinary epibolic
 +
gastrula; but the front and hind ends of the streaks remain practically
 +
stationary. Owing to this mode of growth the edges of the epiblastic cap
 +
and the germinal streaks meet in a linear fashion along the ventral surface
 +
of the embryo (fig. 159, A and B). The germinal streaks first meet anteriorly
 +
(B) and their junction is then gradually continued backwards. The process
 +
is completed at about the time of hatching.
 +
 +
During the above changes the nuclei of the vitelline spheres pass to the
 +
surface and rapidly divide. Eventually, together with part of the protoplasm
 +
of the vitelline spheres, they appear to give rise to a layer of hypoblastic
 +
cells. This layer encloses the remains of the vitelline spheres, which
 +
become the yolk.
 +
 +
At the front end of the germinal streaks, in a position corresponding with that
 +
of the four original epiblast cells,
 +
two depressions appear which
 +
coalesce to form the single oral
 +
invagination ; in the centre of
 +
which are formed the mouth and
 +
pharynx by a second epiblastic
 +
invagination.
 +
 +
The most important point in FIG. 159. Two EMBRYOS OF CLEPSINE IN
 +
 +
connection with the above history WHICH THE GERMINAL STREAKS HAVE PARTIis the fate of what have been ALLY^MET ALONG THE VENTRAL LINE. (After
 +
 +
called the germinal streaks. Ac- ^ germinal, i.e. mesoblastic streaks,
 +
 +
cording to Whitman they are The area covered by epiblast is shaded .
 +
 +
composed of two kinds of cells, The so-called neuroblasts at the end of the
 +
viz. four rows of smaller super- germinal streaks are shewn in B.
 +
ficial cells, which he calls neuroblasts, and, in the later stages at any rate, a
 +
row of deeper large cells, which he calls mesoblasts. As to the eventual fate
 +
of these cells he states that the neuroblasts uniting together in the median
 +
line form the rudiment of the ventral ganglionic chain, while the mesoblasts
 +
equally coalesce and give rise to the mesoblast. Such a mode of origin for a
 +
ventral ganglionic chain is, so far as I know, without a parallel in the whole
 +
animal kingdom ; and whatever evidence Whitman may have that the cells
 +
 +
1 According to Robin it is more usual for there to be only a triple row of primary
 +
neuroblasts.
 +
 +
 +
 +
 +
DISCOPHORA. 349
 +
 +
 +
 +
in question really do give rise to the nervous system he has not thought fit to
 +
produce it in his paper. He figures a section with the eight neuroblastic cells
 +
in the middle ventral line, and in the next stage described the nervous
 +
system is divided up into ganglia ! The first stage, in which the so-called
 +
nervous system has the form of a single row of eight cells, is quite unlike
 +
any rudiment of the nervous system such as is usually met with in the
 +
Chaetopoda, and not a single stage between this and a ganglionated cord is
 +
described or figured. Whitman, whose views seem to have been influenced
 +
by a peculiar, and in my opinion erroneous, theory of Rauber's about the
 +
relation of the neural groove of Vertebrata to the blastopore, does not seem
 +
to be aware that his determination of the fate of his neuroblasts requires any
 +
special support.
 +
 +
He quotes the formation of these parts in Euaxes (vide preceding
 +
Chapter, p. 324) as similar to that in Clepsine. In this comparison it
 +
appears to me probable that he may be quite correct, but the result of the
 +
comparison would be to shew that the neuroblasts and mesoblasts composed
 +
together a mesoblastic band similar to that of the Oligochaeta. Till more
 +
evidence is brought forward by Whitman or some other observer in support
 +
of the view that the so-called neuroblasts have any share in forming the
 +
nervous system, they must in my opinion be regarded as probably forming,
 +
in conjunction with the mesoblasts, two simple mesoblastic bands. Kowalevsky has moreover briefly stated that he has satisfied himself that the
 +
nervous system in Clepsine originates from the epiblast a statement which
 +
certainly could not be brought into harmony with Whitman's account.
 +
 +
Nephelis. Nephelis will form my type of the Gnathobdellidae. The
 +
segmentation of this form has not yet been thoroughly investigated, but
 +
Biitschli's (No. 359) observations are probably the most trustworthy.
 +
 +
The ovum first divides into two, and then into four segments of which
 +
two are slightly smaller than the others. Four small cells which form the
 +
commencement of the epiblast are now formed. Three of them are derived
 +
by budding from the two larger and one of the smaller of the four cells,
 +
and the fourth from a subsequent division of one of the larger cells 1 .
 +
The three cells which assisted in the formation of the epiblast cells again
 +
give rise each to a small cell ; and the small cells so formed constitute a
 +
layer underneath the epiblast which is the commencement of the hypoblast,
 +
while the cells from which they originated form the vitelline spheres.
 +
Shortly after the formation of the hypoblast, the large sphere which has
 +
hitherto been quiescent divides into two, one of which then gives rise
 +
in succession to two small epiblastic elements.
 +
 +
The two large spheres, resulting from the division of the originally
 +
quiescent sphere, next divide again on the opposite side of the embryo,
 +
and form a layer of epiblast there ; so that there is now on one side of
 +
the embryo (the ventral according to Robin) a layer of epiblast formed
 +
 +
1 Doubts have been cast by Whitman on the above account of the origin of the
 +
four epiblast cells.
 +
 +
 +
 +
350 CLEPSINE.
 +
 +
 +
 +
of six cells, and on the opposite side a layer formed of four cells. The
 +
two layers meet at the front border of the embryo and between them are
 +
placed the three large vitelline spheres. The two patches of epiblast cells
 +
now rapidly increase, and gradually spread over the three large vitelline
 +
spheres. Except where they meet -each other at the front edge they leave
 +
uncovered a large part of the margin of the vitelline spheres.
 +
 +
While these changes have been taking place on the exterior, the
 +
hypoblast cells have increased in number (additional cells being probably
 +
derived from the three large vitelline spheres) and fill up in a column-like
 +
fashion a space which is bounded behind by the three vitelline spheres, and
 +
in front by the epiblast of the anterior end of the embryo. At the sides of
 +
the hypoblast the mesoblast has become established, probably as two lateral
 +
bands. The origin of the cells forming it has not yet been determined.
 +
The hypoblast cells in the succeeding stage arrange themselves round a
 +
central archenteric cavity, and at the same time rapidly increase in size
 +
and become filled with a secondary deposit of food-yolk. Shortly afterwards a mouth and thick-walled oesophagus are formed, probably from an
 +
epiblastic invagination. The mesoblast now forms two curved lateral
 +
bands at the two sides of the body, equivalent to the mesoblastic bands
 +
of the Chaetopoda. The three vitelline spheres, still largely uncovered by
 +
the epiblast, lie at the posterior end of the body. The embryo grows
 +
rapidly, especially anteriorly, and the three vitelline spheres become
 +
covered by a layer of flattened epiblast cells. Around the oesophagus a
 +
cavity traversed by muscular fibres is established. Elsewhere there is no
 +
trace of such a cavity. The cephalic region becomes ciliated, and the
 +
dorsal part of it, which represents a rudimentary prae-oral lobe, is especially
 +
prominent. The cilia of the oral region are continued into the lumen of
 +
the oesophagus, and at a later period are prolonged, as in Lumbricus, along
 +
the median line of the ventral surface.
 +
 +
The mesoblastic bands would seem from Biitschli's observations, which
 +
receive confirmation from Kleinenberg's researches on Lumbricus, to be prolonged dorsally to the oesophagus into the cephalic region. Posteriorly they
 +
abut on the large vitelline spheres, which were supposed by Kowalevsky
 +
to give origin to them, and to play the same part as the large mesoblasts in Lumbricus. It has already been shewn that the function of the
 +
large cells in Lumbricus has been exaggerated, and Biitschli denies to
 +
them in Nephelis any share in the production of the mesoblast. It seems
 +
in fact probable that they are homologous with the three vitelline spheres
 +
of Clepsine ; and that their primitive function is to give origin to the
 +
hypoblast. They are visible for a long time at the hind end of the embryo,
 +
but eventually break up into smaller cells, the fate of which is unknown.
 +
 +
The embryo of Hirudo would appear from the researches of Robin
 +
to develop in nearly the same way as that of Nephelis. The anterior
 +
part is not however ciliated. The three large posterior cells disappear
 +
relatively early.
 +
 +
 +
 +
DISCOPHORA. 351
 +
 +
 +
 +
General history of the larva.
 +
 +
The larva of Clepsine, at the time when the mesoblastic
 +
bands have met along the ventral line, is represented in fig.
 +
158 B. It is seen to be already segmented, the process having
 +
proceeded pari passu with the ventral coalescence of the mesoblastic bands. The segments are formed from before backwards
 +
as in Chaetopoda. The dorsal surface is flat and short, and the
 +
ventral very convex. The embryo about this time leaves its
 +
capsule, and attaches itself to its parent. It rapidly elongates,
 +
and the dorsal surface, growing more rapidly than the ventral,
 +
becomes at last the more convex. Eventually thirty-three postoral segments become formed ; of which the eight last coalesce
 +
to form the posterior sucker.
 +
 +
The general development of the body of Nephelis and
 +
Hirudo is nearly the same as that of Clepsine. The embryo
 +
passes from a spherical to an oval, and then to a vermiform
 +
shape. For full details the reader is referred to Robin's
 +
memoir.
 +
 +
The presence of a well-marked protuberance above the
 +
oesophagus, which forms the rudiment of a prae-oral lobe, has
 +
already been mentioned as characteristic of the embryo of
 +
Nephelis ; no such structure is found in Clepsine.
 +
 +
History of the germinal layers and development of organs.
 +
 +
The epiblast. The epiblast is formed of a single layer of
 +
cells and early develops a delicate cuticle which is clearly formed
 +
quite independently of the egg membrane. It becomes raised
 +
into a series of transverse rings which bear no relation to the
 +
true somites of the mesoblast.
 +
 +
The nervous system. The nervous system is probably
 +
derived from the epiblast, but its origin still requires further
 +
investigation. The ventral cord breaks up into a series of
 +
ganglia, which at first correspond exactly with the somites of
 +
the mesoblast. Of these, four or perhaps three eventually coalesce to form the sub-cesophageal ganglion, and seven or eight
 +
become united in the posterior sucker.
 +
 +
It would appear from Biitschli's statements that the supra
 +
 +
 +
352 NEPHELIS.
 +
 +
 +
 +
cesophageal ganglion arises, as in Oligochaeta, independently of
 +
the ventral cord.
 +
 +
Mesoblast. It has already been indicated that the mesoblast probably takes its origin both in Nephelis and Clepsine
 +
from the two mesoblastic bands which unite in the median
 +
ventral line. The further history of these bands is only imperfectly known. They become segmented from before backwards. The somites formed by the segmentation gradually
 +
grow upwards and meet in the dorsal line. Septa are formed
 +
between the somites probably in the same way as in the
 +
Oligochaeta.
 +
 +
In Clepsine the mesoblastic bands are stated by Kowalevsky to become split into somatic and splanchnic layers, between which are placed
 +
the so-called lateral sinuses. These sinuses form, according to Whitman,
 +
a single continuous tube investing the alimentary tract ; a tube which
 +
differs therefore to a very small extent from the normal body cavity of
 +
the Chaetopoda. The somatic layer of mesoblast no doubt gives rise to
 +
the circular and longitudinal muscular layers of the embryo. The former
 +
is stated to appear the earliest, while the latter, as in the Oligochaeta,
 +
first takes its origin on the ventral side.
 +
 +
A delicate musculature, formed mainly of transverse but also of longitudinal fibres, would appear to be developed independently of the mesoblastic bands in Nephelis and Hirudo (Rathke, Leuckart, Robin, and
 +
Biitschli). It develops apparently from certain stellate cells which are
 +
found between the walls of the alimentary tract and the skin, and which
 +
probably correspond to the system of contractile fibres which pass from
 +
the body wall to the alimentary tract through the segmentation cavity in
 +
the larva of Chaetopoda, various Vermes and Mollusca 1 .
 +
 +
The mesoblast, so far as is known, gives rise, in addition to
 +
the parts already mentioned, to the excretory organs, generative
 +
organs, vascular system, etc.
 +
 +
Excretory organs. There are found in the embryo of Nephelis and Hirudo certain remarkable provisional excretory organs
 +
the origin and history of which is not yet fully made out. In
 +
Nephelis they appear as one (according to Robin, No. 364), or
 +
(according to Biitschli, No. 359) as two successive pairs of
 +
 +
1 According to Robin this system of muscles becomes gradually strengthened and
 +
converted into the permanent system. Rathke on the other hand states that it is
 +
provisional, and that it is replaced by the muscles developed from the mesoblastic
 +
somites. It is possible to suppose that it may really become incorporated in the latter
 +
system.
 +
 +
 +
 +
DISCOPHORA. 353
 +
 +
 +
 +
convoluted tubes on the dorsal side of the embryo, which are
 +
stated by the latter author to develop from the scattered mesoblast cells underneath the skin. At their fullest development
 +
they extend, according to Robin, from close to the head to near
 +
the ventral sucker. Each of them is U-shaped, with the open
 +
end forwards, each limb of the U being formed by two tubes
 +
united in front. No external opening has been clearly made
 +
out. Semper believed that the tubes were continuous with
 +
the three posterior vitelline cells, but this has been shewn not
 +
to be the case. Fiirbringer 1 is inclined from his own researches to believe that they open laterally. They contain a
 +
clear fluid.
 +
 +
In Hirudo, Leuckart (No. 362) has described three similar
 +
pairs of organs the structure of which he has fully elucidated.
 +
They are situated in the posterior part of the body, and each of
 +
them commences with an enlargement from which a convoluted
 +
tube is continued for some distance backwards ; it then turns
 +
forwards again and afterwards bends upon itself to open to the
 +
exterior. The anterior part is broken up into a kind of labyrinthic network.
 +
 +
The true segmental organs are found in a certain number
 +
of the segments and are stated (Whitman) to develop from
 +
groups of mesoblast cells. Their origin requires however further
 +
investigation.
 +
 +
A double row of colossal cells on each side of the body has been
 +
described in Clepsine by Whitman as derived from the mesoblastic plates.
 +
These cells (fig. 58 B), which he calls segment-cells, lie opposite the walls of
 +
the septa. The inner row is stated to be connected with the segmental
 +
organs. Their eventual history is unknown, but they are conjectured
 +
by Whitman to be the mother cells of the testes.
 +
 +
The alimentary tract. This is formed primitively of two
 +
parts the epiblastic stomodaeum forming mouth, pharynx,
 +
and oesophagus, and the hypoblastic mesenteron. The anus is
 +
formed very late as a simple perforation immediately dorsal to
 +
the posterior sucker.
 +
 +
In Clepsine, where there is an epibolic gastrula, the rudiment
 +
 +
1 Morphologisches Jahrbuch, Vol. iv. p. 676. He further speaks of the tube as
 +
" feinverzweigt u. netzformig verastelt," but whether from his own observations is
 +
not clear.
 +
 +
B. II. 23
 +
 +
 +
 +
354 DEVELOPMENT OF ORGANS.
 +
 +
of the mesenteron is at first formed of the three vitelline
 +
spheres, from the surface of which a true hypoblastic layer
 +
enclosing a central yolk mass becomes differentiated, as already
 +
described. The mesenteric sack so formed is constricted by the
 +
growth of the mesoblastic septa into a series of lobes, while the
 +
posterior part forms a narrow and at first very short tube opening by the anus.
 +
 +
The lobed region forms the sacculated stomach of the adult.
 +
The sacculations of the stomach by their mode of origin necessarily correspond with the segments. In the adult however the
 +
anterior lobe is really double and has two divisions for the two
 +
segments it fills, while the posterior lobe, which, as is well
 +
known, extends backwards parallel with the rectum, is composed
 +
of five segmental sacculations. In connection with the stomodaeum a protrusible pharynx is developed.
 +
 +
In Hirudo and Nephelis the mesenteron has from the first a
 +
sack-like form. The cells which compose the sack give rise to a
 +
secondary deposit of food-yolk. The further changes are practically the same as in Clepsine. In Hirudo the posterior sacculation of the stomach is primitively unpaired. The jaws are
 +
formed at about the same time as the eyes as protuberances on
 +
the wall of the oral cavity.
 +
 +
BIBLIOGRAPHY.
 +
 +
(359) O. Biitschli. " Entwicklungsgeschichtliche Beitrage (Nephelis)." Zeit.
 +
f. wiss. Zool. Vol. xxix. 1877.
 +
 +
(360) E. Grube. Untersuchnngen iib. d. Entwicklung d. Aniiclidcu. Konigsl)crg, 1844.
 +
 +
(361) C.K.Hoffmann. " Zur Entwicklungsgeschichte d. Clepsineen." Niederldnd. Archiv f. Zool. Vol. iv. 1877.
 +
 +
(362) R. Leuckart. Die mcnschlichen Parasiten (Hirudo), Vol. i. |>. 686,
 +
et seq.
 +
 +
(363) II. Rathke. Beit. z. Entwicklungsgesch. d. Hirudineen. Leipzig, 1862.
 +
 +
(364) Ch. Robin. Mfm. sur le Dhjeloppcment embryogenique des Hirudwccs.
 +
1'aris, 1875.
 +
 +
(365) C. O. Whitman. " Embryology of Clepsine." Quart. J. of Micro.
 +
Science, Vol. xvm. 1878.
 +
 +
[Vide also C. Semper (No. 355) and Kowalevsky (No. 342) for isolated observations.]
 +
 +
 +
 +
CHAPTER XIV.
 +
 +
GEPHYREA 1 .
 +
 +
IT is convenient for the purposes of embryology to divide
 +
the Gephyrea into two groups, viz. (i) Gephyrea nuda or true
 +
Gephyrea; and (2) Gephyrea tubicola formed by the genus
 +
Phoronis.
 +
 +
GEPHYREA NUDA.
 +
Segmentation and formation of the layers.
 +
 +
An embolic or epibolic gastrula is characteristic of the
 +
Gephyrea, and the blastopore appears, in some cases at any rate
 +
(Phascolosoma, Thalassema), to become the mouth.
 +
 +
Bonellia. In Bonellia (Spengel, No. 370) the segmentation
 +
is unequal but complete, and, as in many Molluscs etc., the
 +
ovum exhibits before its commencement a distinction into a
 +
protoplasmic and a yoke pole. The ovum first divides into four
 +
equal segments, each of them formed of the same constituents as
 +
the original ovum. At the animal pole four small cells, entirely
 +
formed of protoplasm, are next formed by an equatorial furrow.
 +
They soon place themselves in the intervals between the large
 +
spheres. Four small cells are again budded off from the large
 +
spheres and the eight small cells then divide. By a further
 +
continuation of the division of the existing small cells, and the
 +
formation of fresh ones from the large spheres, a layer of small
 +
 +
1 The following scheme shews the classification of the Gephyrea adopted in the
 +
present chapter :
 +
 +
 +
 +
i. Gephyrea nuda. {
 +
ii. Gephyrea tubicola (Phoronis).
 +
 +
232
 +
 +
 +
 +
356
 +
 +
 +
 +
SEGMENTATION.
 +
 +
 +
 +
cells is eventually formed, which completely envelops the four
 +
large spheres except for a small blastopore at the vegetative pole
 +
of the ovum (fig. 160 A). The large spheres continue to give
 +
rise to smaller cells which however no longer take a superficial
 +
position but lie within the layer of small cells, and give rise to
 +
the hypoblast (fig. r6o B). The small cells become the epiblast,
 +
and at the blastopore they curl inwards (fig. 160 B) and give
 +
 +
 +
 +
 +
 +
FIG. 160. EPIBOLIC GASTRULA OF BONELLIA. (After Spengel.)
 +
 +
A. Stage when the four hypoblast cells are nearly enclosed.
 +
 +
B. Stage after the formation of the mesoblast has commenced by an infolding of
 +
the lips of the blastopore.
 +
 +
ep. epiblast ; me. mesoblast ; bl. blastopore.
 +
 +
rise to a layer of cells, which appears to extend as an unbroken
 +
sheet between the epiblast and hypoblast, and to form the
 +
mesoblast. The blastopore now closes up, but its position in
 +
relation to the parts of the embryo has not been made out.
 +
 +
In Phascolosoma (Selenka, No. 369) the ovum, enclosed in a
 +
porous zona radiata, divides into two unequal spheres, of which
 +
the smaller next divides into two and then into four. An
 +
invagination takes place which is intermediate between the
 +
embolic and the epibolic types. The small cells, the number of
 +
which is increased by additions from the large sphere, divide, and
 +
grow round the large sphere. The latter in the meantime also
 +
divides, and the cells produced from it form on the one hand a
 +
small sack which opens by the blastopore, and on the other they
 +
fill up the segmentation cavity, and become the mesoblast and
 +
blood corpuscles. Tin- Mastoporc becomes the permanent
 +
mouth.
 +
 +
 +
 +
GEPHYREA.
 +
 +
 +
 +
357
 +
 +
 +
 +
Larval forms and development of organs.
 +
 +
Amongst the Gephyrea armata the larva has as a rule
 +
(Thalassema, Echiurus) the characters of a trochosphere, and
 +
closely approaches the typical form characteristic of the larva of
 +
Polygordius, often known as Loven's larva. In Bonellia this
 +
larval form is less perfectly preserved.
 +
 +
Echiurus. In Echiurus (Salensky, No. 368) the youngest
 +
known larva has all the typical trochosphere characters (fig. 161).
 +
It is covered with cilia and divided into a prae-oral lobe and
 +
post-oral region of nearly equal dimensions. There is a double
 +
ciliated ring which separates the two sections of the body as in
 +
the larva of Polygordius : the mouth (m) opens between its two
 +
elements. The alimentary canal is divided into a stomodaeum
 +
with a ventral opening, a large stomach, and a short intestine
 +
opening by a terminal anus (an). Connecting the oesophagus
 +
with the apex of the prae-oral lobe is the usual contractile band,
 +
and at the insertion of this band -is a thickening of the epiblast
 +
which probably represents the rudiment of the supra-oesophageal
 +
ganglion. A ventral nerve cord is stated by Salensky to be
 +
present, but his observations on this point are not quite satisfactory.
 +
 +
The metamorphosis is accompanied by the loss of swimming
 +
power, and consists in the
 +
enlargement of the post-oral
 +
portion of the trunk, and in
 +
the simultaneous reduction
 +
of the prae-oral lobe, which
 +
remains however permanently as the cylindrical
 +
proboscis. A groove which
 +
terminates posteriorly at the
 +
mouth is very early formed
 +
on its ventral side. The
 +
ciliated rings gradually disappear during the metamorphosis.
 +
 +
 +
 +
 +
FIG. 161. LARVA OF ECHIURUS.
 +
(After Salensky.)
 +
 +
 +
 +
_ m. mouth ; an. anus ; sg. supra-ceso
 +
 +
Of the further external phageal ganglion (?).
 +
 +
 +
 +
358
 +
 +
 +
 +
LARVAL FORMS.
 +
 +
 +
 +
changes the most important are (i) the early appearance
 +
round the anal end of the body of a ring of bristles ; and (2) the
 +
appearance of a pair of ventral setae in the anterior part of the
 +
body. The anterior ring of bristles characteristic of the adult
 +
Echiurus does not appear till a late period.
 +
 +
Of the internal changes the earliest is the formation of the
 +
anal respiratory sacks. With the growth of the posterior part
 +
of the trunk the intestine elongates, and becomes coiled.
 +
 +
Bonellia. The embryo of Bonellia, while still within the
 +
egg, retains a spherical form and acquires an equatorial band of
 +
cilia, behind which a second narrower band is soon established,
 +
while in front of the first one a pair of eye-spots becomes
 +
 +
 +
 +
 +
KM;. 162. THREE STAGES IN THE DEVELOPMENT OF BONELLIA. (After Spengel.)
 +
 +
A. Larva with two ciliated bands and two eye-spots.
 +
 +
B. Ripe larva from the dorsal surface.
 +
 +
C. Young female Bonellia from the side.
 +
 +
a/, alimentary tract ; m. mouth ; sc. provisional excretory tube ; s. ventral hook ;
 +
an.-', anal vesicle.
 +
 +
formed (fig. 162 A). The embryo on becoming hatched rapidly
 +
elongates, while at the same time it becomes dorso-vcntrally
 +
flattened and acquires a complete coating of cilia (fig. 162 B).
 +
According to Spengel it resembles at this time in its form and
 +
habits a rhabdoccelous Turbcllarian. The anterior part is
 +
however somewhat swollen and presents an indication of a
 +
pre-oral lobe.
 +
 +
 +
 +
GEPHYREA. 359
 +
 +
 +
 +
During the above changes important advances are made in the formation of the organs from the embryonic layers.
 +
 +
The epiblast acquires a superficial cuticula, which is perhaps directly
 +
derived from the vitelline membrane. The nervous system is also formed,
 +
probably from the epiblast. The band-like supra-cesophageal ganglion is
 +
the first part of the nervous system formed, and appears to be undoubtedly
 +
derived from the epiblast. The ventral cord arises somewhat later, but the
 +
first stages in its development have not been satisfactorily traced. It is
 +
continuous with the supra-cesophageal band which completely girths the
 +
oesophagus without exhibiting any special dorsal enlargement. After the
 +
ventral cord has become completely separated from the epiblast a central
 +
fibrous mass becomes differentiated in it, while the lateral parts are composed
 +
of ganglion cells. In the arrangement of its cells it presents indications of
 +
being composed of two lateral halves. It is, however, without ganglionic
 +
swellings.
 +
 +
The mesoblast, though at first very thin, soon exhibits a differentiation
 +
into a splanchnic and somatic layer though the two do not become
 +
distinctly separated by a body cavity. The somatic layer rapidly becomes
 +
thicker, and enlarges laterally to form two bands united dorsally and
 +
ventrally by narrow, thinner bands. The outermost parts of each of these
 +
bands become differentiated into an external circular and an internal
 +
longitudinal layer of muscles. In the pras-oral lobe the mesoblast assumes
 +
a somewhat vacuolated character.
 +
 +
The hypoblast cells form a complete layer round the four yolk cells from
 +
which they arise (fig. 162 B, al\ but at first no alimentary lumen is developed.
 +
The oesophagus appears during this period as an, at first solid, but subsequently hollow, outgrowth of the hypoblast towards the epiblast.
 +
 +
The metamorphosis of the larva into the adult female
 +
Bonellia commences with the conversion of many of the indifferent mesoblast cells into blood corpuscles, and the introduction into the body cavity of a large amount of fluid, which
 +
separates the splanchnic and somatic layers of mesoblast. The
 +
fluid is believed by Spengel to be sea-water, introduced by two
 +
anal pouches, the development of which is described below.
 +
 +
The body cavity is lined by a peritoneum, and very soon
 +
distinct vessels, formed by folds of the peritoneum, become
 +
established. Of these there are three trunks, two lateral and a
 +
median in the prae-oral lobe (proboscis), and in the body a
 +
ventral trunk above the nerve cord, and an intestinal trunk
 +
opening anteriorly into the ventral one. The vessels appear
 +
to communicate with the body cavity.
 +
 +
In the course of the above changes the two ciliated bands
 +
 +
 +
 +
360 LARVAL FOR. MX
 +
 +
 +
 +
disappear, the hinder one first. The cilia covering the general
 +
surface become atrophied, with the exception of those on the
 +
ventral side of the prae-oral lobe. The latter structure becomes
 +
more prominent ; the stellate mesoblast cells, which fill up its
 +
interior, become contractile, and it gives rise to the proboscis
 +
(fig. 162 C).
 +
 +
At the point where the cesophageal protuberance joined the epiblast at
 +
a previous stage the mouth becomes established (fig. 162 C, ;//), and though
 +
it is formed subsequently to the atrophy of the anterior ciliated band, yet
 +
there is evidence that it is potentially situated behind this band. The lumen
 +
of the alimentary canal becomes established by the absorption of the
 +
remains of the four central cells. The anus is formed on the ventral side
 +
of the posterior end of the body, and close to it the pouches already noticed
 +
grow out from the hindermost part of the alimentary tract (fig. 162 C, an.v\
 +
They are at first simple blind pouches, but subsequently open into the
 +
body cavity 1 . They become the anal pouches of the adult. There is present
 +
when the mouth is first formed a peculiar process of the alimentary tract
 +
projecting into the prae-oral lobe, which appears to atrophy shortly afterwards.
 +
 +
After the formation of the mouth, there are formed on the ventral side of
 +
and slightly behind it (i) anteriorly a pair of tubes, which appear to be
 +
provisional excretory organs and soon disappear (fig. 162 C, sc}\ and (2)
 +
behind them a pair of bristles (s) which remain in the adult. The formation
 +
of the permanent excretory (?) organ (oviduct and uterus) has not been
 +
followed out. The ovary appears very early as a differentiation of the
 +
epithelium lining the ventral vessel.
 +
 +
The larvae, which become the minute parasitic males, undergo
 +
a very different and far less complete metamorphosis than those
 +
which become females. They attach themselves to the proboscis of an adult female, and lose their ciliated bands. Germinal cells make their appearance in the mesoblast, which form
 +
spherical masses, and, like the germinal balls in the female
 +
ovary, consist of a central cell, and an epithelium around it.
 +
The central cell becomes very large, while the peripheral cells
 +
give rise to the spermatozoa. A body cavity becomes developed
 +
in the larvae, into which the spermatic balls are dehisced.
 +
Neither mouth nor anus is formed. The further changes have
 +
not been followed out.
 +
 +
1 The fact that these pouches are outgrowths of the alimentary tract appears to
 +
preclude the possibility of their being homologous with excretory tubes of the Plalyelminthes and Rotifera.
 +
 +
 +
 +
GEPHYREA. 361
 +
 +
 +
 +
The larval males make their way into the oesophagus of the
 +
female, where they no doubt live for some time, and probably
 +
become mature, though the seminal pouch of the adult is not
 +
found in many of the males living in the oesophagus. When
 +
mature the males leave the oesophagus, and pass into the
 +
uterus.
 +
 +
Phascolosoma. Cilia appear in Phascolosoma (Selenka,
 +
No. 369) while the ovum is still segmenting. After segmentation
 +
they form a definite band immediately behind the mouth, which
 +
divides the Jarva into two hemispheres a prae-oral and a postoral. A prae-oral band of cilia is soon formed close to the postoral band, and at the apex of the prae-oral lobe a tuft of cilia
 +
also appears.
 +
 +
The larva has now the characters of a trochosphere, but
 +
differs from the typical trochosphere in the post-oral part of the
 +
ciliated equatorial ring being more important than the prae-oral,
 +
and in the absence of an anus.
 +
 +
The metamorphosis commences very early. The trunk
 +
rapidly elongates, and the prae-oral lobe becomes relatively less
 +
and less conspicuous. The zona radiata becomes the larval
 +
cuticle.
 +
 +
Three pairs of bristles are formed on the trunk, of which the
 +
posterior pair appears first, then the anterior, and finally the
 +
middle pair : an order of succession which clearly proves they
 +
can have no connection with a true segmentation.
 +
 +
The tentacles become developed between the two parts of the
 +
ciliated ring, and finally the prse-oral lobe, unlike what takes
 +
place in the Gephyrea armata, nearly completely vanishes.
 +
 +
The anus appears fairly late on the dorsal surface, and the
 +
ventral nerve cord is established as an unganglionated thickening
 +
of the ventral epiblast.
 +
 +
GEPHYREA TUBICOLA.
 +
 +
The larva of Phoronis was known as Actinotrocha long
 +
before its connection with Phoronis was established by Kowalevsky (No. 372). There is a complete segmentation leading to
 +
the formation of a blastosphere, which is followed by an invagination, the opening of which is said by Kowalevsky to remain as
 +
 +
 +
 +
362 ACTINOTROCHA.
 +
 +
 +
 +
the mouth 1 . It is at first terminal, but on the development of a
 +
large prse-oral lobe it assumes a ventral position. The anus
 +
is formed at a later period at the posterior end of the body.
 +
 +
 +
 +
 +
FlG. 163. A SERIES OF STAGES IN THE DEVELOPMENT OF PlIORONIS FROM
 +
 +
ACTINOTROCHA. (After Metschnikoff.)
 +
 +
A. Young larva.
 +
 +
H. Larva after the formation of post-oral ring of tentacles.
 +
 +
C. Larva with commencing invagination to form the body of Phoronis.
 +
 +
D. Invagination partially everted.
 +
 +
E. Invagination completely everted.
 +
 +
m. mouth ; an. anus; iv. invagination to form the body of Phoronis.
 +
 +
The youngest free larva observed by Metschnikoff (No. 373)
 +
was less developed than the oldest larva found by Kowalevsky.
 +
 +
1 Kowalevsky states that whnt I have called the mouth is the anus, but his subsequent descriptions shew that he has transposed the mouth and anus in the embryo,
 +
and that the opening, which he asserts to be the anus, is in reality the mouth.
 +
 +
 +
 +
GEPHYREA. 363
 +
 +
 +
 +
It probably belongs to a different species. The body is uniformly ciliated (fig. 163 A). There is a large contractile prse-oral
 +
lobe, and the body ends behind in two processes. The mouth
 +
(m) is ventral, and the anus (an) dorsal, and not terminal as in
 +
Kowalevsky's larva.
 +
 +
The alimentary tract is divided into stomodseum, stomach
 +
and intestine. The two processes at the hind end of the body
 +
are the rudiments of the first-formed pair of the arms which are
 +
so characteristic of the fully developed Actinotrocha. A second
 +
pair of arms next become established on the dorsal side of the
 +
previously existing pair, and the region where the anus is placed
 +
grows out as a special process. New pairs of arms continue to
 +
be formed in succession dorsalwards and forwards, and soon
 +
constitute a complete oblique post-oral ring (fig. 163 B). They are
 +
covered by long cilia. Round the anal process a very conspicuous
 +
ciliated ring also becomes established.
 +
 +
At the period when five pairs of arms are present a delicate membrane
 +
becomes visible on the ventral side of the intestine which joins the somatic
 +
mesoblast anteriorly. This membrane is the rudiment of the future ventral
 +
vessel. The somatic mesoblast is present even before this period as a
 +
delicate layer of circular muscular fibres.
 +
 +
When six pairs of arms have become formed an involution
 +
(fig. 163 C, iv) appears on the ventral side, immediately behind
 +
the ring of arms. This involution consists both of the epiblast
 +
and somatic mesoblast. It grows inwards towards the intestine,
 +
and, increasing greatly in length, becomes at the same time
 +
much folded.
 +
 +
When it has reached its full development the critical period
 +
of the metamorphosis of Actinotrocha into Phoronis is reached,
 +
and is completed in about a quarter of an hour. The ventral
 +
involution becomes evoluted (fig. 163 D), just as one might turn
 +
out the finger of a glove which had been pulled inwards. When
 +
the involution has been to a certain extent everted, the alimentary canal passes into it, and at the same time the body of the
 +
larva becomes violently contracted. By the time the evagination is completed it forms (fig. 162 E) a long conical body,
 +
containing the greater part of the alimentary tract, and constituting the body of the young- Phoronis. The original anal process
 +
remains on the dorsal side as a small papilla (fig. 162 E, an).
 +
 +
 +
 +
364 \< T1NOTROCHA.
 +
 +
 +
 +
While these changes have been taking place the prae-oral
 +
lobe has become much contracted, and partly withdrawn into
 +
the stomodajum. At the same time the arms have become bent
 +
forward, so as to form a ring round the mouth. Their bases
 +
become much thickened. The metamorphosis is completed by
 +
the entire withdrawal of the prae-oral lobe within the oesophagus,
 +
and by the casting off of the ends of the arms, their bases
 +
remaining as the circumoral ring of tentacles, which form
 +
however a lophophore rather than a complete ring. The
 +
perianal ring of cilia is also thrown off, and the anal process
 +
withdrawn into the body of the young Phoronis. There are now
 +
three longitudinal vascular trunks, united anteriorly by a circular
 +
vessel which is prolonged into the tentacles.
 +
 +
General Considerations.
 +
 +
The development of Phoronis is so different from that of the
 +
other Gephyrea that further investigations are required to shew
 +
whether Phoronis is a true Gephyrean. Apart from its peculiar
 +
metamorphosis Actinotrocha is a very interesting larval form, in
 +
that it is without a prae-oral ciliated ring, and that the tentacles
 +
of the adult are derived from a true post-oral ring, prolonged
 +
into arm-like processes.
 +
 +
The other Gephyrea present in their development an obvious
 +
similarity to the normal Chaetopoda, but their development stops
 +
short of that of the Chaetopoda, in that they are clearly without
 +
any indications of a true segmentation. In the face of what is
 +
known of their development it is hardly credible that they can
 +
represent a degenerate Chaetopod phylum in which segmentation
 +
has become lost. Further than this the Gephyrea armata seem
 +
in one respect to be a very primitive type in that they retain
 +
through life a well-developed pra-oral lobe, which constitutes
 +
their proboscis. In almost all other forms, except Balanoglossus,
 +
the larval prae-oral lobe becomes reduced to a relatively insignificant anterior part of the head.
 +
 +
BIBLIOGRAPHY.
 +
Gephyrea nuda.
 +
 +
(366) A. Kowalevsky. Sitz. d. zool. Abth. d. III. Vcrsam. russ. Naturj.
 +
(Thalasscma). Zeit.f. wiss. Zool. Vol. xxn. 1872, p. 284.
 +
 +
 +
 +
GEPHYREA. 365
 +
 +
 +
 +
(367) A. Krohn. "Ueb. d. Larve d. Sipunculus nudus nebst Bemerkungen,"
 +
etc. Miiller's Archiv, 1857.
 +
 +
(368) M. Salensky. "Ueber die Metamorphose d. Echiurus." Morphologisches
 +
Jahrbuch, Bd. 11.
 +
 +
(369) E. Selenka. "Eifurchung u. Larvenbildung von Phascolosoma elongatum." Zeit.f. wiss. ZooL 1875, Bd. xxv. p. i.
 +
 +
(370) J. W. Spengel. "Beitrage z. Kenntniss d. Gephyreen (Bonellia)." Mittheil. a. d. zool. Station z. Neapel, Vol. I. 1879.
 +
 +
Gephyrea tubicola (Actinotrocha).
 +
 +
(371) A. Krohn. " Ueb. Pilidium u. Actinotrocha." Miiller's Archiv, 1858.
 +
 +
(372) A. Kowalevsky. "On anatomy and development of Phoronis," Petersbourg, 1867. 2 PI. Russian. Vide Leuckart's Bericht, 1866-7.
 +
 +
(373) E. Metschnikoff. " Ueber d. Metamorphose einiger Seethiere (Actinotrocha)." Zeit.f. wiss. Zool. Bd. xxi. 1871.
 +
 +
(374) J. Miiller. " Bericht lib. ein. Thierformen d. Nordsee." Miiller's Archiv,
 +
1846.
 +
 +
(375) An. Schneider. "Ueb. d. Metamorphose d. Actinotrocha branchiata."
 +
Miiller's Arch. 1862.
 +
 +
 +
 +
CHAPTER XV.
 +
 +
 +
 +
CH/ETOGNATHA, MYZOSTOMEA AND GASTROTRICHA.
 +
 +
THE present chapter deals with three small isolated groups,
 +
which only resemble each other in that the systematic position of
 +
all of them is equally obscure.
 +
 +
Chatognatha.
 +
 +
The discoveries of Kowalevsky (No. 378) confirmed by
 +
Btitschli (No. 376) with reference to the development of Sagitta,
 +
though they have not brought us nearer to a knowledge of the
 +
systematic position of this remarkable form, are nevertheless of
 +
 +
 +
 +
 +
FIG. 164. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C after
 +
Hiitschli and B after Kowalevsky.) The three embryos are represented in the same
 +
positions.
 +
 +
A. The gastrula stage.
 +
 +
li. A succeeding stage in which the primitive archenteron is commencing to be
 +
divided into three parts, the two lateral of which are destined to form the body
 +
cavity.
 +
 +
C. A later stage in which the mouth involution (/;/) has become continuous with
 +
the alimentary tract, and the blastopore has become closed.
 +
 +
m. mouth; al. alimentary canal ; ae. archenteron ; bl.p. blastopore; pv. perivisceral
 +
cavity; sf>. splanchnopleuric mesoblast; so. somatopleuric mesoblast ; ge. generative
 +
organs.
 +
 +
 +
 +
CH^TOGNATHA. 367
 +
 +
 +
 +
great value for the more general problems of embryology. The
 +
development commences after the eggs are laid. The segmentation is uniform, and a blastosphere, formed of a single layer of
 +
columnar cells, is the product of it. An invagination takes
 +
place, the opening of which narrows to a blastopore situated
 +
at the pole of the embryo opposite that at which the mouth
 +
subsequently appears (fig. 164 A). The simple archenteron soon
 +
becomes anteriorly divided into three lobes, which communicate
 +
freely with the still single cavity behind (fig. 164 B). The two
 +
lateral lobes are destined to form the body cavity, and the
 +
median lobe the alimentary tract of the adult. An invagination
 +
soon arises at the opposite pole of the embryo to the blastopore
 +
and forms the mouth and oesophagus (fig. 164 B and C, m).
 +
 +
At the gastrula stage there is formed a paired mass destined
 +
to give rise to the generative organs. It arises as a prominence
 +
of six cells, projecting from the hypoblast at the anterior pole of
 +
the archenteron, and soon separates itself as a mass, or probably
 +
a pair of masses, lying freely in the cavity of the archenteron
 +
(fig. 164 A. y ge). When the folding of the primitive cavity takes
 +
place the generative rudiment is situated at the hind end of the
 +
median lobe of the archenteron in the position represented
 +
in fig. 164 C, ge.
 +
 +
An elongation of the posterior end of the embryo now takes
 +
place, and the embryo becomes coiled up in the egg, and when
 +
eventually hatched sufficiently resembles the adult to be recognisable as a young Sagitta.
 +
 +
Before hatching takes place various important changes
 +
become manifest. The blastopore disappears after being carried
 +
to the ventral surface. The middle section of the trilobed region
 +
of the archenteron becomes separated from the unpaired
 +
posterior part, and forms a tube, blind behind, but opening
 +
in front by the mouth (fig. 165 A, al). It constitutes the permanent alimentary tract, and is formed of a pharyngeal epiblastic
 +
invagination, and a posterior hypoblastic section derived from
 +
the primitive archenteron. The anus is apparently not formed
 +
till comparatively late. After the isolation of the alimentary
 +
tract the remainder of the archenteron is formed of two cavities
 +
in front, which open freely into a single cavity behind (fig.
 +
165 A). The whole of it constitutes the body cavity and its walls
 +
 +
 +
 +
3 68
 +
 +
 +
 +
CH/ETOGNATHA.
 +
 +
 +
 +
f/ic mesoblast. The anterior paired part becomes partitioned off
 +
into a head section and a trunk section (fig. 165 A and B). The
 +
former constitutes a pair of distinct cavities (c.pv) in the head,
 +
and the latter two cavities opening freely into the unpaired
 +
portion behind. At the junction of the paired cavities with the
 +
unpaired cavity are situated the generative organs (ge). The
 +
inner wall of each of the paired cavities forms the splanchnopleuric mesoblast, and the outer wall of the whole the somatic
 +
mesoblast. The inner walls of the posterior cavities unite above
 +
and below the alimentary tract, and form the dorsal and ventral
 +
mesenteries, which divide the body cavity into two compartments
 +
in the adult. Before the hatching of the embryo takes place
 +
this mesentery is continued backwards so as to divide the
 +
primitively unpaired caudal part of the body cavity in the
 +
same way.
 +
 +
From the somatic mesoblast of the trunk is derived the
 +
single layer of longitudinal muscles of Sagitta, and part of the
 +
epithelioid lining of the body cavity. The anterior termination
 +
of the trunk division of the body cavity is marked in the adult
 +
by the mesentery dividing into two laminae, which bend outwards
 +
to join the body wall.
 +
 +
The cephalic section of the body cavity
 +
seems to atrophy, and
 +
its walls to become converted into the complicated system of muscles
 +
present in the head of
 +
the adult Sagitta.
 +
 +
In the presence of
 +
a section of the body
 +
cavity in the head the
 +
embryo of Sagitta re
 +
sembles Lumbricus,
 +
Spiders, etc.
 +
 +
The generative rudiment of each side
 +
divides into an anterior
 +
and a posterior part
 +
 +
 +
 +
 +
In;. [65. Two VIEWS OF A LATE EMBRYO OF
 +
SV.ITTA. A. from the dorsal surface. I?, from the
 +
tide. (After 15iitschli.)
 +
 +
m. mouth ; al. alimentary canal ; v.g. ventral
 +
ganglion (thickening of epiblast) ; rp. epiblast ; c.pv,
 +
cephalic section of body cavity; so. somatopleure ;
 +
s/>. splanchnopleure ; ,;v. generative
 +
 +
 +
 +
CH^ETOGNATHA. 369
 +
 +
 +
 +
(fig. 165, ge]. The former constitutes the ovary, and is situated
 +
in front of the septum dividing the tail from the body ; and the
 +
latter, in the caudal region of the trunk, forms the testis.
 +
 +
The nervous system originates from the epiblast. There is a
 +
ventral thickening (fig. 165 B, v.g) in the anterior region of the
 +
trunk, and a dorsal one in the head. The two are at first
 +
continuous, and on becoming separated from the epiblast remain
 +
united by thin cords.
 +
 +
The ventral ganglion is far more prominent during embryonic
 +
life than in the adult. Its position and early prominence in the
 +
embryo perhaps indicate that it is the homologue of the ventral
 +
cord of Chaetopoda 1 .
 +
 +
BIBLIOGRAPHY.
 +
 +
(376) O. Biitschli. "Zur Entwicklungsgeschichte der Sagitta." Zeitschrift f.
 +
wiss. Zoo!., Vol. xxni. 1873.
 +
 +
(377) C. Gegenbaur. " Uber die Entwicklung der Sagitta." Abhand. d. naturforschenden Gesellschaft in Halle, 1857.
 +
 +
(378) A. Kowalevsky. " Embryologische Studien an Wiirmern u. Arthropoden." Mem. Acad. Petersbourg, VII. ser., Tom. XVI., No. 12. 1871.
 +
 +
MYZOSTOMEA.
 +
 +
The development of these peculiar parasites on Crinoids has been
 +
investigated by Metschnikoff (No. 380), Semper (No. 381), and Graff
 +
(No. 379).
 +
 +
The segmentation is unequal, and would appear to be followed by an
 +
epibolic invagination. The outer layer of cells (epiblast) becomes covered
 +
with cilia, and the inner is transformed into a non-cellular (?) central yolk
 +
mass. At this stage the larva is hatched, and commences to lead a free
 +
existence. In the next stage observed by Metschnikoff, the mouth, oesophagus, stomach, and anus had become developed ; and two pairs of feet
 +
were present. In both of these feet Chaetopod-like setae were present, which
 +
in the hinder pair were simple fine bristles without a terminal hook. The
 +
papilliform portion of the foot is at first undeveloped. The feet become
 +
successively added, like Chaetopod segments, and the stomach does not
 +
become dendriform till the whole complement of feet (5 pairs) are present.
 +
 +
In the primitive covering of cilia, combined with a subsequent indication
 +
 +
1 Langerhans has recently made some important investigations on the nervous
 +
system of Sagitta, and identifies the ventral ganglion with the parieto-splanchnic ganglia of Molluscs, while he has found a pair of new ganglia, the development of which
 +
is unknown, which he calls the suboesophageal or pedal ganglia. The embryological facts do not appear to be in favour of these interpretations.
 +
 +
B. II. 24
 +
 +
 +
 +
3/0 MYZOSTOMEA.
 +
 +
 +
 +
of segments in the formation of the feet and setae, the larva of the Myzostomea shews an approximation to the Chaetopoda, and the group is
 +
probably to be regarded as an early Chactopod type specially modified in
 +
connection with its parasitic habits.
 +
 +
BIBLIOGRAPHY.
 +
 +
(379) L.Graff. Das Genus Myzostoma. Leipzig, 1877.
 +
 +
(380) E. Metschnikoff. "Zur Entwicklungsgeschichte d. Myzostomum."
 +
Zfit.f. wiss. Zool. y Vol. XVI. 1866.
 +
 +
(381) C. Semper. "Z. Anat. u. Entwick. d. Gat. Myzostomum." Ztit.f. wiss.
 +
Zool., Vol. ix. 1858.
 +
 +
GASTROTRICHA.
 +
 +
A few observations of Ludwig on the winter eggs of Ichthydium larus
 +
shew that the segmentation is a total and apparently a regular one. It
 +
leads to the formation of a solid morula. The embryo has a ventral
 +
curvature, and the caudal forks are early formed as cuticular structures. By
 +
the time the embryo leaves the egg, it has almost reached the adult state.
 +
The ventral cilia arise some little time prior to the hatching.
 +
 +
BIBLIOGRAPHY.
 +
 +
(382) H. Ludwig. " Ueber die Ordnung Gastrotricha Mctschn" Zeit. f. wiss.
 +
Zool., Vol. xxvi. 1876.
 +
 +
 +
 +
CHAPTER XVI.
 +
 +
NEMATELMINTHES AND ACANTHOCEPHALA.
 +
 +
NEM ATELMINTHES '.
 +
 +
Nematoidea. Although the ova of various Nematodes have
 +
formed some of the earliest, as well as the most frequent objects
 +
of embryological observation, their development is still but very
 +
imperfectly known. Both viviparous and oviparous forms are
 +
common, and in the case of the oviparous forms the eggs are
 +
usually enveloped in a hard shell. The segmentation is total
 +
and nearly regular, though the two first segments are often
 +
unequal. The relation of the segmentation spheres to the
 +
germinal layers is however only satisfactorily established (through
 +
the researches of Butschli (No. 383)) in the case of Cucullanus
 +
elegans, a form parasitic in the Perch 2 .
 +
 +
The early development of this embryo takes place within
 +
the body of the parent, and the egg is enveloped in a delicate
 +
membrane. After the completion of the early stages of segmentation the embryo acquires the form of a thin flat plate
 +
composed of two layers of cells (fig. 166 A and B). The two
 +
layers of this plate give rise respectively to the epiblast and
 +
hypoblast, and at a certain stage the hypoblastic layer ceases to
 +
 +
1 The following classification of the Nematoda is employed in this chapter :
 +
 +
r Ascaridae.
 +
Strongylidae.
 +
 +
Trichinidse. II. Gordioidea.
 +
 +
I. Nematoidea. , Filarid8B . m . Chaetosomoidea.
 +
 +
Mermithidae.
 +
[_ Anguillulidse.
 +
 +
2 The ova of Anguillula aceti are stated by Hallez to undergo a similar development to those of Cucullanus.
 +
 +
242
 +
 +
 +
 +
372
 +
 +
 +
 +
CUCULLANUS.
 +
 +
 +
 +
grow, while the growth of the epiblastic layer continues. As a
 +
consequence of this the sides of the plate begin to fold over
 +
towards the side of the hypoblast (fig. 166 D.) This folding
 +
results in the formation of a remarkably constituted gastrula,
 +
which has the form of a hollow two-layered cylinder with an
 +
incompletely closed slit on one side (fig. 166 E, bl.p}. This slit
 +
has the value of a blastopore. It becomes closed by the coalescence of the two edges, a process which commences posteriorly,
 +
 +
 +
 +
 +
FIG. 166.
 +
 +
 +
 +
A.
 +
B.
 +
C.
 +
 +
 +
 +
VARIOUS STAGES IN THE DEVELOPMENT OF CUCULLANUS ELEGANS.
 +
 +
(From Biitschli.)
 +
 +
Surface view of flattened embryo at an early stage in the segmentation.
 +
Side view of an embryo at a somewhat later stage, in optical section.
 +
Flattened embryo at the completion of segmentation.
 +
 +
D. Embryo at the commencement of the gastrula stage.
 +
 +
E. Embryo when the blastopore is reduced to a mere slit.
 +
 +
F. Vermiform embryo after the division of the alimentary tract into oesophageal
 +
and glandular divisions.
 +
 +
m. mouth; ep. epiblast; hy. hypoblast; me. mesoblast; a?, oesophagus; bl.p. blastopore.
 +
 +
and then gradually extends forwards. In front the blastopore
 +
never becomes completely closed, but remains as the permanent
 +
mouth. The embryo after these changes has a worm-like form,
 +
which becomes the more obvious as it grows in length and
 +
becomes curved (fig. 166 F).
 +
 +
The hypoblast of the embryo gives rise to the alimentary
 +
 +
 +
 +
NEMATELM1NTHES. 373
 +
 +
 +
 +
canal, and soon becomes divided into an cesophageal section
 +
(fig. 1 66 F, ce) formed of granular cells, and a posterior division
 +
formed of clear cells. The mesoblast (fig. 166, me) takes its
 +
origin from certain special hypoblast cells around the mouth,
 +
and thence grows backwards towards the posterior end of the
 +
body.
 +
 +
The young Cucullanus becomes hatched while still in the
 +
generative ducts of its parent, and is distinguished by the
 +
presence of a remarkable thread-like tail. On the dorsal surface
 +
is a provisional boring apparatus in the form of a conical papilla.
 +
A firm cuticle enveloping the body is already present. In this
 +
condition it leaves its parent and host, and leads for a time a
 +
free existence in the water. Its metamorphosis is dealt with in
 +
another section.
 +
 +
The ova of the Oxyuridae parasitic in Insects are stated by Galeb (No.
 +
386) to take the form of a blastosphere at the close of segmentation. An
 +
inner layer is then formed by delamination. What the inner layer gives rise
 +
to is not clear, since the whole alimentary canal is stated to be derived from
 +
two buds, which arise at opposite ends of the body, and grow inwards till
 +
they meet.
 +
 +
The generative organs. The study of the development of
 +
the generative organs of Nematodes has led to some interesting
 +
results. In the case of both sexes the generative organs originate (Schneider, No. 390) from a single cell. This cell elongates
 +
and its nuclei multiply. After assuming a somewhat columnar
 +
form, it divides into (i) a superficial investing layer, and (2) an
 +
axial portion.
 +
 +
In the female the superficial layer is only developed distinctly in the median part of the column. In the course of the
 +
further development the two ends of the column become the
 +
blind ends of the ovary, and the axial tissue they contain forms
 +
the germinal tissue of nucleated protoplasm. The superficial
 +
layer gives rise to the epithelium of the uterus and oviduct.
 +
The germinal tissue, which is originally continuous, is interrupted
 +
in the middle part (where the superficial layer gives rise to the
 +
uterus and oviduct), and is confined to the two blind extremities
 +
of the tube.
 +
 +
In the male the superficial layer, which gives rise to the epithelium of the vas deferens, is only formed at the hinder end of
 +
 +
 +
 +
374 METAMORPHOSIS.
 +
 +
 +
 +
the original column. In other respects the development takes
 +
place as in the female.
 +
 +
Gordioidea. The ovum of Gordius undergoes a regular segmentation.
 +
According to Villot (No. 391) it forms at the close of segmentation a morula,
 +
which becomes two-layered by delamination. The embryo is at first
 +
spherical, but soon becomes elongated.
 +
 +
By an invagination at the anterior extremity the head is formed. It
 +
consists of a basal portion, armed with three rings of stylets, and a conical
 +
proboscis, armed with three large stylets. When the larva becomes free
 +
the head becomes everted, though it remains retractile. By the time the
 +
embryo is hatched a complete alimentary tract is formed with an oral opening
 +
at the end of the proboscis, and a subterminal ventral anal opening. It is
 +
divided into an oesophagus and stomach, and a large gland opens into it at
 +
the base of the proboscis.
 +
 +
The body has a number of transverse folds, which give it a ringed
 +
appearance.
 +
 +
Metamorphosis and life history.
 +
 +
Nematoidea. Although a large number of Nematodes have
 +
a free existence and simple life history, yet the greater number
 +
of known genera are parasitic, and undergo a more or less complicated metamorphosis 1 . According to this metamorphosis
 +
they may be divided into two groups (which by no means
 +
closely correspond with the natural divisions), viz. those which
 +
have a single host, and those with two hosts. Each of these
 +
main divisions may be subdivided again into two.
 +
 +
In the first group with one host the simplest cases are those
 +
in which the adult sexual form of parasite lays its eggs in the
 +
alimentary tract of its host, and the eggs are thence transported
 +
to the exterior. The embryo still in the egg, if favoured by
 +
sufficient warmth and moisture, completes its development up
 +
to a certain point, and, if then swallowed by an individual of the
 +
species in which it is parasitic in the adult condition, it is
 +
denuded of its shell by the action of the gastric juice, and
 +
develops directly into the sexual form.
 +
 +
Leuckart has experimentally established this metamorphosis in the case
 +
of Trichocephalus affinis, Oxyurus ambigua, and Heterakis vermicularis.
 +
The Oxyuridae of Blatta and Hydrophilus have a similar life history
 +
 +
1 The following facts are mainly derived from Leuckart's exhaustive treatise
 +
(No. 388).
 +
 +
 +
 +
NEMATELMINTHES. 375
 +
 +
 +
 +
(Caleb, No. 386), and it is almost certain that the metamorphosis of the
 +
human parasites, Ascaris lumbricoides and Oxyurus vermicularis, is of this
 +
nature.
 +
 +
A slightly more complicated metamorphosis is common in
 +
the genera Ascaris and Strongylus. In these cases the egg-shell
 +
is thin, and the embryo becomes free externally, and enjoys for
 +
a shorter or longer period a free existence in water or moist
 +
earth. During this period it grows in size, and though not
 +
sexual usually closely resembles the adult form of the permanently free genus Rhabditis. In some cases the free larva
 +
becomes parasitic in a freshwater Mollusc, but without thereby
 +
undergoing any change. It eventually enters the alimentary
 +
tract of its proper host and there become sexual.
 +
 +
As examples of this form of development worked out by Leuckart may
 +
be mentioned Uochmius trigonocephalus, parasitic in the dog, and Ascaris
 +
acuminata, in the frog. The human parasite Dochmius duodenale undergoes the same metamorphosis as Dochmius trigonocephalus.
 +
 +
A remarkable modification of this type of metamorphosis is found in
 +
Ascaris (Rhabdonema) nigrovenosa, which in its most developed condition
 +
is parasitic in the lungs of the frog (Metschnikoff, Leuckart, No. 388). The
 +
embryos pass through their first developmental phases in the body of the
 +
parent. They have the typical Rhabditis form, and make their way after
 +
birth into the frog's rectum. From this they pass to the exterior, and then
 +
living either in moist earth, or the faeces of the frog, develop into a sexual
 +
form, but are very much smaller than in the adult condition. The sexes are
 +
distinct, and the males are distinguished from the females by their smaller
 +
size, shorter and rounded tails, and thinner bodies. The females have
 +
paired ovaries with a very small number of eggs, but the testis of the males
 +
is unpaired. Impregnation takes place in the usual way, and in summer
 +
time about four embryos are developed in each female, which soon burst
 +
their egg-capsules, and then move freely in the uterus. Their active movements soon burst the uterine walls, and they then come to lie freely in the
 +
body cavity. The remaining viscera of the mother are next reduced to a
 +
finely granular material, which serves for the nutrition of the young forms
 +
which continue to live in the maternal skin. The larvae eventually become
 +
free, and though in many respects different from the parent form which gave
 +
rise to them, have nevertheless the Rhabditis form. They live in water or
 +
slime, and sometimes become parasitic in water-snails ; in neither case however do they undergo important changes unless eventually swallowed by a
 +
frog. They then pass down the trachea into the lungs and there rapidly
 +
develop into the adult form. No separate males have been found in the
 +
lungs of the frog, but it has been shewn by Schneider (No. 390) that the
 +
so-called females are really hermaphrodites ; the same gland giving origin
 +
 +
 +
 +
376 METAMORPHOSIS.
 +
 +
 +
 +
to both spermatozoa and ova, the former being developed before the latter 1 .
 +
The remarkable feature of the above life history is the fact that in the stage
 +
corresponding with the free larval stage of the previous forms the larvae of
 +
this species become sexual, and give rise to a second free larval generation,
 +
which develops into the adult form on again becoming parasitic in the
 +
original host. It constitutes a somewhat exceptional case of heterogamy as
 +
defined in the introduction.
 +
 +
Amongst the Nematodes with but a single host a remarkable parasite in
 +
wheat has its place. This form, known as Anguillula scandens, inhabits in
 +
the adult condition the ears of wheat, in which it lays its eggs. After
 +
hatching, the larvae become encysted, but become free on the death of the
 +
plant. They now inhabit moist earth, but eventually make their way into
 +
the ears of the young wheat and become sexually mature.
 +
 +
The second group of parasitic Nematodes with two hosts
 +
may be divided into two groups, according to whether the larva
 +
has a free existence before passing into its first or intermediate
 +
host, or is taken into it while still in the egg. In the majority
 +
of cases the larval forms live in special connective tissue capsules, or sometimes free in the tissues of their intermediate
 +
hosts ; but the adults, as in the cases of other parasitic Nematodes, inhabit the alimentary tract.
 +
 +
The life history of Spiroptera obtusa may be cited as an example of a
 +
Nematode with two hosts in which the embryo is transported into its
 +
intermediate host while still within the egg. The adult of this form is
 +
parasitic in the mouse, and the ova pass out of the alimentary tract with the
 +
excreta, and may commonly be found in barns, etc. If one of the ova is
 +
now eaten by the meal-worm (larva of Tenebrio), it passes into the body
 +
cavity of this worm and undergoes further development. After about five
 +
weeks it becomes encapsuled between the ' fat bodies ' of the meal-worm.
 +
It then undergoes an ecdysis, and, if the meal-worm with its parasites is
 +
now eaten by the mouse, the parasites leave their capsule and develop into
 +
the sexual form.
 +
 +
As examples of life histories in which a free state intervenes before the
 +
intermediate host, Cucullanus elegans and Dracunculus may be selected.
 +
The adult Cucullanus elegans is parasitic in the alimentary tract of the Perch
 +
and other freshwater fishes. It is a viviparous form, and the young after
 +
birth pass out into the water. They next become parasitic in Cyclops,
 +
passing in through the mouth, so into the alimentary tract, and thence into
 +
the body cavity. They soon undergo an ecdysis, in the course of which the
 +
oesophagus becomes divided into a muscular pharynx and true glandular
 +
 +
1 Leuckart does not appear to be satisfied as to the hermaphroditism of these
 +
forms ; and holds that it is quite possible that the ova may develop parthenogenetically.
 +
 +
 +
 +
NEMATELMINTHES. 377
 +
 +
 +
 +
oesophagus. They then grow rapidly in length, and at a second ecdysis
 +
acquire a peculiar beaker-like mouth cavity approaching that of the adult.
 +
They do not become encapsuled. No further development of the worm
 +
takes place so long as it remains in the Cyclops, but, if the Cyclops is now
 +
swallowed by a Perch, the worm undergoes a further ecdysis, and rapidly
 +
attains to sexual maturity.
 +
 +
The observations of Fedschenko on Dracunculus medinensis 1 , which is
 +
parasitic in the subcutaneous connective tissue in Man, would seem to shew
 +
that it undergoes a metamorphosis very similar to that of Cucullanus. There
 +
is moreover a striking resemblance between the larvae of the two forms.
 +
The larvae of Dracunculus become transported into water, and then make
 +
their way into the body cavity of a Cyclops by boring through the soft skin
 +
between the segments on the ventral surface of the body. In the body cavity
 +
the larvae undergo an ecdysis and further development. But on reaching
 +
a certain stage of development, though they remain a long time in the
 +
Cyclops, they grow no further. The remaining history is unknown, but
 +
probably the next host is man, in which the larva comes to maturity. In the
 +
adult condition only females of Dracunculus are known, and it has been
 +
suggested by various writers that the apparent females are in reality hermaphrodites, like Ascaris nigrovenosa, in which the male organs come to
 +
maturity before the female.
 +
 +
Another very remarkable human parasite belonging to the same group
 +
as Dracunculus is the form known as Filaria sanguinis hominis, or Filaria
 +
Bancrofti 2 .
 +
 +
The sexual form is parasitic in warm climates in the human tissues, and
 +
produces multitudes of larvae which pass into the blood, and are sometimes
 +
voided with the urine. The larvae in the blood do not undergo a further development, and unless transported to an intermediate host die before very long.
 +
Some, though as yet hardly sufficient, evidence has been brought forward to
 +
shew that if the blood of an infected patient is sucked by a mosquito the
 +
larvae develop further in the alimentary tract of the mosquito, pass through a
 +
more or less quiescent stage, and eventually grow considerably in size, and
 +
on the death of the mosquito pass into the water. From the water they are
 +
probably transported directly or indirectly into the human intestines, and
 +
then bore their way into the tissues in which they are parasitic, and become
 +
sexually mature.
 +
 +
The well-known Trichina spiralis has a life history unlike that of other
 +
known Nematodes, though there can be little doubt that this form should
 +
be classified in respect to its life history with the last- described forms.
 +
The peculiarity of the life history of Trichina is that the embryos set free
 +
in the alimentary canal pass through the walls into the muscular tissues and
 +
there encyst ; but do not in a general way pass out from the alimentary
 +
 +
1 Vide Leuckart, D. men. Par., Vol. II. p. 704.
 +
 +
2 Vide D. P. Manson, " On the development of Filaria sanguinis hominis."
 +
Journal of the Linnean Society, Vol. xiv. No. 75.
 +
 +
 +
 +
378 MKTAMORPHOSIS.
 +
 +
 +
 +
canal of one host and thence into a fresh host to encyst. It occasionally
 +
however happens that this migration does take place, and the life history
 +
of Trichina spiralis then becomes almost identical with that of some of the
 +
forms of the third type. Trichina is parasitic in man, and in swine, and
 +
also in the rat, mouse, cat, fox and other forms which feed upon them.
 +
Artificially it can be introduced into various herbivorous forms (rabbit,
 +
guinea-pig, horse) and even birds.
 +
 +
The sexual form inhabits the alimentary canal. The female is viviparous, and produces myriads of embryos, which pass into the alimentary
 +
canal of their host, through the walls of which they make their way, and
 +
travelling along lines of connective tissue pass into the muscles. Here the
 +
embryos, which are born in a very imperfect condition, rapidly develop,
 +
and eventually assume a quiescent condition in a space inclosed by sarcolemma. Within the sarcolemma a firm capsule is developed for each larva,
 +
which after some months becomes calcified ; and after the atrophy of the
 +
sarcolemma a connective tissue layer is formed around it. Within its
 +
capsule the larva can live for many years, even ten or more, without
 +
undergoing further development, but if at last the infected flesh is eaten by
 +
a suitable form, e.g. the infected flesh of the pig by man, the quiescent
 +
state of the larva is brought to a close, and sexual maturity is attained in
 +
the alimentary tract of the new host.
 +
 +
Gordioidea. The free larva of Gordius already described usually penetrates into the larva of Chironomus where it becomes encysted. On the
 +
Chironomus being eaten by some fish (Villot, No. 39) (Phoxinus laevis or
 +
Cobitis barbatula), it penetrates into the wall of the intestine of its second
 +
host, becomes again encysted and remains quiescent for some time. Eventually in the spring it leaves its capsule, and enters the intestine, and
 +
passes to the exterior with the faeces. It then undergoes a gradual metamorphosis, in the course of which it loses its ringed structure and cephalic
 +
armature, grows in length, acquires its ventral cord, and on the development of the generative organs loses the greater part of its alimentary tract.
 +
 +
Young examples of Gordius have often been found in various terrestrial
 +
carnivorous Insecta, but the meaning of this fact is not yet clear.
 +
 +
 +
 +
BIBLIOGRAPHY.
 +
 +
(383) O. Biitschli. "Entwicklungsgeschichte d. Cucullanus elegans." Zdt.j.
 +
wiss. Zool., B. xxvi. 1876.
 +
 +
(384) T. S. Cobbold. Entozoa. Groombridge and Son, 1864.
 +
 +
(385) T. S. Cobbold. Parasites; A Treatise on the Entozoa of Man mn/
 +
Animals. Churchill, 1879.
 +
 +
(386) O. Galeb. "Organisation et developpement des Oxyurides," &c. Archives de Zool. expcr. et getter. , Vol. vn. 1878.
 +
 +
(387) R. Leu ck art. Untcrsufkutigcn itb. Trichina spiralis. 2nd ed. Leip/ig,
 +
1866.
 +
 +
(388) R. Leuckart. Die tnenschlichcn Parasitcn, Bd. II. 1876.
 +
 +
 +
 +
NEMATELMINTHES. 379
 +
 +
 +
 +
(389) H. A. Pagenstecher. Die Trichinen nach Versitchen dargestellt. Leipzig, 1865.
 +
 +
(390) A.Schneider. Monographic d. Nemaioden. Berlin, 1866.
 +
 +
(391) A. Villot. "Monographic des Dragoneaux" (Gordioidea). Archives de
 +
Zool. exper. et gener., Vol. ill. 1874.
 +
 +
ACANTHOCEPHALA.
 +
 +
The Acanthocephala appear to be always viviparous. At the time of
 +
impregnation the ovum is a naked cell, and undergoes in this condition the
 +
earlier phases of segmentation.
 +
 +
The segmentation is unequal (Leuckart, No. 393), but whether there is an
 +
epibolic gastrula has not clearly been made out.
 +
 +
Before segmentation is completed there are formed round the ovum
 +
thick protecting membranes, which are usually three in number, the middle
 +
one being the strongest. After segmentation the central cells of the ovum
 +
fuse together to give rise to a granular mass, while the peripheral cells at a
 +
slightly later period form a more transparent syncytium. At the anterior
 +
end of the embryo there appears a superficial cuticle bearing in front a ring
 +
of hooks.
 +
 +
The embryo is now carried out with the excreta from the intestine of
 +
the vertebrate host in which its parent lives. It is then swallowed by some
 +
invertebrate host 1 .
 +
 +
In the intestine of the invertebrate host the larva is freed from its
 +
membranes, and is found to have a somewhat elongated conical form, terminating anteriorly in an obliquely placed disc, turned slightly towards the
 +
ventral surface and armed with hooks. Between this disc and the granular
 +
mass, already described as formed from the central cells of the embryo, is a
 +
rather conspicuous solid body. Leuckart supposes that this body may represent a rudimentary functionless pharynx, while the granular mass in
 +
his opinion is an equally rudimentary and functionless intestine. The body
 +
wall is formed of a semifluid internal layer surrounding the rudimentary
 +
intestine, if such it be, and of a firmer outer wall immediately within the cuticle.
 +
The adult Echinorhyncus is formed by a remarkable process of development within the body of the larva, and the skin is the only part of the
 +
larva which is carried over to the adult.
 +
 +
In Echinorhyncus proteus the larva remains mobile during the formation of the adult, but in other forms the metamorphosis takes place during
 +
a quiescent condition of the larva.
 +
 +
The organs of the adult are differentiated from a mass of cells which
 +
appears to be a product of the central embryonic granular mass, and is
 +
 +
1 Echin. proteus, which is parasitic in the adult state in many freshwater fish,
 +
passes through its larval condition in the body cavity of Gammarus pulex. Ech.
 +
angustatus, parasitic in the Perch, is found in the larval condition in the body cavity
 +
of Asellus aquaticus. Ech. gigas, parasitic in swine, is stated by Schneider (No. 394)
 +
to pass through its larval stages in maggots.
 +
 +
 +
 +
380 ACANTHOCEPHALA.
 +
 +
 +
 +
called by Leuckart the embryonic nucleus. The embryonic nucleus becomes
 +
divided into four linearly arranged groups of cells, of which the hindermost
 +
but one is the largest, and very early differentiates itself into (i) a peripheral
 +
layer, and (2) a central mass formed of two distinct bodies. The peripheral
 +
layer of this segment grows forwards and backwards, and embraces the
 +
other segments, with the exception of the front end of the first one which
 +
is left uncovered. The envelope so formed gives rise to the splanchnic and
 +
somatic mesoblast of the adult worm. Of the four groups of cells within it
 +
the anterior gives rise to the proboscis, the next to the nerve ganglion, the
 +
third, formed of two bodies, to the paired generatives, and the fourth to the
 +
generative ducts. The whole of the above complex rapidly elongates, and
 +
as it does so the enveloping membrane becomes split into two layers ; of
 +
which the outer forms the muscular wall of the body (somatic mesoblast),
 +
and the inner the muscular sheath of the proboscis and the so-called generative ligament enveloping the generative organs. The inner layer may be
 +
called the splanchnic mesoblast in spite of the absence of an intestine.
 +
The cavity between the two mesoblastic layers forms the body cavity.
 +
 +
The various parts of the adult continue to differentiate themselves as
 +
the whole increases in size. The generative masses very early shew traces
 +
of becoming differentiated into testes or ovaries. In the male the two
 +
generative masses remain spherical, but in the female become elongated :
 +
the rudiment of the generative ducts becomes divided into three sections
 +
in both sexes. The most remarkable changes are, however, those undergone
 +
by the rudiment of the proboscis.
 +
 +
In its interior there is formed a cavity, but the wall bounding the front
 +
end of the cavity soon disappears. By the time that this has taken place
 +
the body of the adult completely fills up the larval skin, to which it very
 +
soon attaches itself. The hollow rudiment of the proboscis then becomes
 +
everted, and forms a papilla at the end of the body, immediately adjoining the larval skin. This papilla, with the larval skin covering it,
 +
constitutes the permanent proboscis. The original larval cuticle is either
 +
now or at an earlier period thrown off and a fresh cuticle developed. The
 +
hooks of the proboscis are formed from cells of the above papilla, which
 +
grow through the larval skin as conical prominences, on the apex of which
 +
a chitinous hook is modelled. The remainder of the larval skin forms the
 +
skin of the adult, and at a later period develops in its deeper layer the
 +
peculiar plexus of vessels so characteristic of the Acanthocephala. The
 +
anterior oval appendages of the adult cutis, known as the lemnisci, are
 +
outgrowths from the larval skin.
 +
 +
The Echinorhyncus has with the completion of these changes practically
 +
acquired its adult structure ; but in the female the ovaries undergo at this
 +
period remarkable changes, in that they break up into a number of spherical
 +
masses, which lie in the lumen of the generative ligaments, and also make
 +
their way into the body cavity.
 +
 +
The young Echinorhyncus requires to be transported to its permanent
 +
host, which feeds on its larval host, before attaining to sexual maturity.
 +
 +
 +
 +
ACANTHOCEPHALA. 381
 +
 +
 +
 +
BIBLIOGRAPHY.
 +
 +
(392) R. Greeff. " Untersuchungen ii. d. Bau u. Entwicklung des Echin. miliarius." Archiv f. Naturgesch. 1864.
 +
 +
(393) R. Leuckart. Die menschlichen Parasiten. Vol. n. p. 80 1 et seq.
 +
1876.
 +
 +
(394) An. Schneider. " Ueb. d. Bau d. Acanthocephalen." Archiv f. Anat.
 +
u, Phys. 1868.
 +
 +
(395) G. R. Wagener. Beitrdge z. Entwicklungsgeschichte d. Eingeweidewiirmer. Haarlem, 1865.
 +
 +
 +
 +
CHAPTER XVII.
 +
 +
TRACHEATA.
 +
 +
PROTOTRACH EAT A.
 +
THE remarkable researches of Moseley (No. 396) on Peripatus
 +
 +
 +
 +
 +
FIG. 167. ADULT EXAMPLE OF PERIPATUS CAPENSIS, natural size.
 +
(From Moseley.)
 +
 +
capensis have brought clearly to light the affinities of this form
 +
with the tracheate Arthropoda ; and its numerous primitive
 +
 +
 +
 +
 +
FIG. 168. Two STAGES IN THE DEVELOPMENT OF PERIPATUS CAPENSIS.
 +
(After Moseley.)
 +
 +
A. Youngest stage hitherto observed before the appearance of the legs.
 +
 +
B. Later stage after the legs and antennae have become developed.
 +
Both figures represent the larva as it appears within the egg.
 +
 +
i and i. First and second post-oral appendages.
 +
 +
characters, such as the generally distributed tracheal apertures,
 +
the imperfectly segmented limbs, the diverging ventral nerve
 +
 +
 +
 +
TRACHEATA.
 +
 +
 +
 +
383
 +
 +
 +
 +
cords with imperfectly marked ganglia, and the nephridia (segmental organs 1 ), would render its embryology of peculiar interest. Unfortunately Moseley was unable, from want of
 +
material, to make so complete a study of its development as of
 +
its anatomy. The youngest embryo observed was in part
 +
distinctly segmented, and coiled up within the egg (fig. 168 A).
 +
The procephalic lobes resemble those of the Arthropoda generally, and are unlike the prae-oral lobe of
 +
Chaetopods or Discophora. They are not
 +
marked off by a transverse constriction
 +
from the succeeding segments. The three
 +
embryonic layers are differentiated, and
 +
the interior is filled with a brownish mass
 +
the remnant of the yolk which is probably enclosed in a distinct intestinal wall,
 +
and is lobed in correspondence with the
 +
segmentation of the body. The mouth
 +
invagination is not present, and but two
 +
pairs of slight prominences mark the rudiments of the two anterior post-oral appendages.
 +
 +
The single pair of antennae is formed
 +
in the next stage, and is followed by the
 +
remaining post-oral appendages, which
 +
arise in succession from before backwards
 +
somewhat later than the segments to which
 +
they appertain.
 +
 +
The posterior part of the embryo becomes uncoiled, and the whole embryo
 +
bent double in the egg (fig. 168 B).
 +
 +
The mouth appears as a slit-like opening between and below the procephalic
 +
lobes. On each side and somewhat behind it there grows out
 +
an appendage the first post-oral pair (fig. 169, i) while in
 +
front and behind it are formed the upper and lower lips. These
 +
two appendages next turn inwards towards the mouth, and their
 +
 +
 +
 +
 +
FIG. 169. EMBRYO
 +
OF PERIPATUS CAPENSIS.
 +
Slightly older than A in
 +
fig. 168; unrolled. (After
 +
Moseley.)
 +
 +
a. antennae ; o. mouth ;
 +
i. intestine ; c. procephalic
 +
lobe, i, 2, 3, etc., postoral appendages.
 +
 +
 +
 +
1 F. M. Balfour, "On certain points in the Anatomy of Peripatus capensis."
 +
Quart. Journ. of Micros. Science, Vol. xix. 1879.
 +
 +
 +
 +
PROTOTRACHEATA.
 +
 +
 +
 +
 +
bases become gradually closed over by two processes of the
 +
procephalic region (fig. 170, m)
 +
The whole of these structures
 +
assist in forming a kind of
 +
secondary mouth cavity, which
 +
is at a later period further
 +
completed by the processes of
 +
the procephalic region meeting
 +
above the mouth, covering over
 +
the labrum, and growing backwards to near the origin of the
 +
second pair of post-oral appendages.
 +
 +
The antennae early become
 +
jointed, and fresh joints continue to be added throughout
 +
embryonic life ; in the adult
 +
there are present fully thirty
 +
joints. It appears to me probable (though Mr Moseley takes
 +
the contrary view) from the late development of the paired
 +
processes of the procephalic lobes, which give rise to the circular
 +
lip of the adult, that they
 +
are not true appendages.
 +
The next pair therefore
 +
to the antennae is the first
 +
post-oral pair. It is the
 +
only pair connected with
 +
the mouth. At their extremities there is formed a
 +
pair of claws similar to
 +
those of the ambulatory
 +
 +
legs (fig. 171). The next FIG. 171. HEAD OF AN EMBRYO PERIPA, . . r TUS. (From Moseley.)
 +
 +
and largest pair of appen- The figure shews the jaws (mamlil)lcs)> and
 +
 +
dagCS in the embryo are close to them epiblastic involutions, which
 +
 +
 +
 +
FIG. 170. VENTRAL VIEW OF THE
 +
HEAD OF AN EMBRYO OF PERIPATUS CAPENSIS AT A LATE STAGE OF DEVELOPMENT.
 +
 +
/. thickening of epiblast of procephalic lobe to form supra-oesophageal ganglion ; ///. process from procephalic lobe
 +
growing over the first post-oral appendage ; o. mouth; e. eye; i and 2, first
 +
and second pair of post-oral appendages.
 +
 +
 +
 +
 +
the oral papillae. They
 +
 +
 +
 +
grow into the supra-oesophageal ganglia. The
 +
antennae, oral cavity, and oral papilhe are also
 +
 +
are chiefly remarkable for shewn.
 +
 +
containing the ducts of the slime glands which open at their
 +
bases. They are without claws. The succeeding appendages
 +
become eventually imperfectly five-jointed ; two claws are
 +
 +
 +
 +
TRACHEATA.
 +
 +
 +
 +
385
 +
 +
 +
 +
formed as cuticular investments of papillae in pockets of the
 +
skin at the ends of their terminal joints.
 +
 +
I have been able to make a few observations on the internal structure of
 +
the embryos from specimens supplied to me by Moseley. These are so far
 +
confined to a few stages, one slightly earlier, the others slightly later, than
 +
the embryo represented in fig. 168 B. The epiblast is formed of a layer of
 +
columnar cells, two deep on the ventral surface, except along the median line
 +
where there is a well-marked groove and the epiblast is much thinner (fig. 172).
 +
 +
The ventral cords of the trunk are formed as two independent epiblastic
 +
thickenings. In my earlier stage these are barely separated from the
 +
epiblast, but in the later ones are quite independent (fig. 172, v.n), and
 +
partly surrounded by mesoblast.
 +
 +
The supra-cesophageal ganglia are formed as thickenings of the epiblast
 +
of the ventral side of the procephalic lobes in front of the stomodaeum.
 +
They are shewn at / in fig. 170. The thickenings of the two sides are at
 +
first independent. At a somewhat later period an invagination of the
 +
epiblast grows into each of these lobes. The openings of these invaginations
 +
extend from the oral cavity forwards; and they are shewn in fig. 171 l .
 +
Their openings become closed, and the walls of the invaginations constitute
 +
a large part of the embryonic supra-cesophageal ganglia.
 +
 +
Similar epiblastic invaginations assist in forming the supra-cesophageal
 +
ganglia of other Tracheata.
 +
They are described in the sequel
 +
for Insects, Spiders and Scorpions. The position of the supracesophageal ganglia on the ventral side of the procephalic lobes
 +
is the same as that in other
 +
Tracheata.
 +
 +
The mesoblast is formed, in
 +
the earliest of my embryos, of
 +
scattered cells in the fairly wide
 +
space between the mesenteron
 +
and the epiblast. There are two
 +
distinct bands of mesoblast on
 +
the outer sides of the nervous
 +
cords. In the later stage the
 +
mesoblast is divided into distinct somatic and splanchnic layers, both very thin ; but the two
 +
layers are connected by transverse strands (fig. 172). There
 +
 +
 +
 +
sp.w
 +
 +
 +
 +
$.m
 +
 +
 +
 +
 +
FIG. 172. SECTION THROUGH THE TRUNK
 +
OF AN EMBRYO OF PERIPATUS. The embryo
 +
from which the section is taken was somewhat
 +
younger than fig. 171.
 +
 +
sp.m. splanchnic mesoblast.
 +
 +
s.m. somatic mesoblast.
 +
 +
me. median section of body cavity.
 +
 +
k. lateral section of body cavity.
 +
 +
v.n. ventral nerve cord.
 +
 +
me. mesenteron.
 +
 +
 +
 +
1 This figure is taken from Moseley. The epiblastic invaginations are represented
 +
in it very accurately, and though not mentioned in the text of the paper, Moseley
 +
informs me that he has long been aware of the homology of these folds with those in
 +
various other Tracheata.
 +
 +
 +
 +
B. II.
 +
 +
 +
 +
2 5
 +
 +
 +
 +
386 PROTOTRACHEATA.
 +
 +
 +
 +
are two special longitudinal septa dividing the body cavity into three
 +
compartments, a median (me), containing the mesenteron, and two lateral
 +
(Ic) containing the nerve cords. This division of the body cavity persists,
 +
as I have elsewhere shewn, in the adult. A similar division is found in
 +
some Chaetopoda, e.g. Polygordius.
 +
 +
I failed to make out that the mesoblast was divided into somites, and
 +
feel fairly confident that it is not so in the stages I have investigated.
 +
 +
There is a section of the body cavity in the limbs as in embryo Myriapods, Spiders, etc.
 +
 +
In the procephalic lobe there is a well-developed section of the body
 +
cavity, which lies dorsal to and in front of the rudiment of the supracesophageal ganglia.
 +
 +
The alimentary tract is formed of a mesenteron (fig. 172), a stomodaeum, and proctodaeum. The wall of the mesenteron is formed, in the
 +
stages investigated by me, of a single layer of cells with yolk particles,
 +
and encloses a lumen free from yolk. The forward extension of the
 +
mesenteron is remarkable.
 +
 +
The stomodaeum in the earlier stage is a simple pit, which meets but does
 +
not open into the mesenteron. In the later stage the external opening of
 +
the pit is complicated by the structures already described. The proctodaeum is a moderately deep pit near the hinder end of the body.
 +
 +
The existence of a tracheal system 1 is in itself almost sufficient to
 +
demonstrate the affinities of Peripatus with the Tracheata, in spite of the
 +
presence of nephridia. The embryological characters of the procephalic
 +
lobes, of the limbs and claws, place however this conclusion beyond
 +
the reach of scepticism. If the reader will compare the figure of Peripatus
 +
with that of an embryo Scorpion (fig. 196 A) or Spider (fig. 200 C) or better
 +
still with Metschnikoffs figure of Geophilus (No. 399) PI. xxi. fig. u,he
 +
will be satisfied on this point.
 +
 +
The homologies of the anterior appendages are not very easy
 +
to determine ; but since there does not appear to me to be sufficient evidence to shew that any of the anterior appendages have
 +
become aborted, the first post-oral appendages embedded in the
 +
lips may provisionally be regarded as equivalent to the mandibles,
 +
and the oral papillae to the first pair of maxillae, etc. Moseley is
 +
somewhat doubtful about the homologies of the appendages,
 +
and hesitates between considering the oral papillae as equivalent
 +
to the second pair of maxillae (on account of their containing the
 +
openings of the mucous glands, which he compares with the
 +
spinning glands of caterpillars), or to the poison claws (fourth
 +
 +
1 The specimens shewing tracheae which Moseley has placed in my hands are
 +
quite sufficient to leave no doubt whatever in my mind as to the general accuracy of
 +
his description of the tracheal system.
 +
 +
 +
 +
TRACHEATA. 387
 +
 +
 +
 +
post-oral appendages) of the Chilopoda (on account of the
 +
poison-glands which he thinks may be homologous with the
 +
mucous glands).
 +
 +
The arguments for either of these views do not appear to me conclusive. There are glands opening into various anterior appendages in
 +
the Tracheata, such as the poison glands in the Chelicerae (mandibles) of
 +
Spiders, and there is some evidence in Insects for the existence of a gland
 +
belonging to the first pair of maxillae, which might be compared with the
 +
mucous gland of Peripatus. For reasons already stated I do not regard
 +
the processes of the cephalic lobes, which form the lips, as a pair of true
 +
appendages.
 +
 +
BIBLIOGRAPHY.
 +
 +
(396) H. N. Moseley. "On the Structure and Development of Peripatus
 +
capensis." Phil. Trans. Vol. 164, 1874.
 +
 +
MYRIAPODA 1 .
 +
 +
Chilognatha. The first stages in the development of the
 +
Chilognatha have been investigated by Metschnikoffand Stecker,
 +
but their accounts are so contradictory as hardly to admit of
 +
reconciliation.
 +
 +
According to Metschnikoff, by whom the following four
 +
species have been investigated, viz., Strongylosoma Guerinii,
 +
Polydesmus complanatus, Polyxenus lagurus, and Julus Moneletei, the segmentation is at first regular and complete, but,
 +
when the segments are still fairly large, the regular segmentation
 +
is supplemented by the appearance of a number of small cells at
 +
various points on the surface, which in time give rise to a
 +
continuous blastoderm.
 +
 +
The blastoderm becomes thickened on the ventral surface,
 +
and so forms a ventral plate 2 .
 +
 +
1 The classification of the Myriapoda employed in the present section is
 +
 +
I. Chilognatha. (Millipedes.)
 +
II. Chilopoda. (Centipedes.)
 +
 +
2 Stecker's (No. 400) observations were made on the eggs of Julus fasciatus, Julus
 +
fcetidus, Craspedosoma marmoratum, Polydesmus complanatus, and Strongylosoma
 +
pallipes, and though carried on by means of sections, still leave some points very
 +
obscure, and do not appear to me deserving of much confidence. The two species of
 +
Julus and Craspedosoma undergo, according to Stecker, a nearly identical development. The egg before segmentation is constituted of two substances, a central protoplasmic, and a peripheral deutoplastic. It first divides into two equal segments, and
 +
coincidentally with their formation part of the central protoplasm travels to the
 +
 +
252
 +
 +
 +
 +
388 CHILOGNATHA.
 +
 +
 +
 +
 +
 +
FIG. 173. THREE STAGES IN THE DEVELOPMENT OF STRONGYLOSOMA GUERINII.
 +
(After Metschnikoff.)
 +
 +
A. Embryo on eleventh day with commencing ventral flexure (*).
 +
 +
B. Embryo with three pairs of post-oral appendages.
 +
 +
C. Embryo with five pairs of post-oral appendages.
 +
 +
gs. ventral plate; at. antenme; 15 post-oral appendages; x. point of flexure of
 +
the ventral plate.
 +
 +
surface as two clear fluid segments. The ovum is thus composed of two yolk segments
 +
to two protoplasmic segments. The two former next divide into four, with the production of two fresh protoplasmic segments. The four protoplasmic segments now
 +
constitute the upper or animal pole of the egg, and occupy the position of the future
 +
ventral plate. The yolk segments form the lower pole, which is however dorsal in
 +
relation to the future animal. The protoplasmic segments increase in number by a
 +
regular division, and arrange themselves in three rows, of which the two outermost
 +
rapidly grow over the yolk segments. A large segmentation cavity is stated to be
 +
present in the interior of the ovum.
 +
 +
It would appear from Stecker's description that the yolk segments (hypoblast)
 +
next become regularly invaginated, so as to enclose a gastric cavity, opening externally
 +
by a blastopore; but it is difficult to believe that a typical gastrula, such as that
 +
represented by Sleeker, really comes into the cycle of development of the Chilognatha.
 +
 +
The mesoblast is stated to be derived mainly from the epiblast. This layer in the
 +
region of the future ventral plate becomes reduced to two rows of cells, and the inner
 +
of these by the division of its constituent elements gives rise to the mesoblast. The
 +
development of Polydesmus and Strongylosoma is not very different from that of Julus.
 +
The protoplasm at the upper pole occupies from the first a superficial position.
 +
Segmentation commences at the lower pole, where the food yolk is mainly present !
 +
The gastrula is stated to be similar to that of Julus, The mesoblast is formed in
 +
Polydesmus as a layer of cells split off from the epiblast, but in Strongylosoma as an
 +
outgrowth from the lips of the blastopore. Stecker, in spite of the statements in his
 +
paper as to the origin of the mesoblast from the epiblast, sums up at the end to the
 +
effect that both the primary layers have a share in the formation of the mesoblast,
 +
which originates by a process of endogenous cell-division !
 +
 +
It may be noted that the closure of the blastopore takes place, according to
 +
Stecker, on the dorsal side of the embryo.
 +
 +
 +
 +
TRACHEATA. 389
 +
 +
 +
 +
The most important sources of information for the general
 +
embryology of the Chilognatha are the papers of Newport (No.
 +
397) and Metschnikoff (No. 398). The development of Strongylosoma may be taken as fairly typical for the group ; and the
 +
subsequent statements, unless the reverse is stated, apply to the
 +
species of Strongylosoma investigated by Metschnikoff.
 +
 +
After the segmentation and formation of the layers the first
 +
observable structure is a transverse furrow in the thickening of
 +
the epiblast on the ventral surface of the embryo. This furrow
 +
rapidly deepens, and gives rise to a ventral flexure of the embryo
 +
(fig. 173 A, x\ which is much later in making its appearance in
 +
Julus than in Strongylosoma and Polyxenus. A pair of appendages, which become the antennae, makes its appearance
 +
shortly after the formation of the transverse furrow, and there
 +
soon follow in order the next three pairs of appendages. All
 +
these parts are formed in the infolded portion of the ventral
 +
thickening of the blastoderm (fig. 173 B). The ventral thickening has in the meantime become marked by a longitudinal
 +
furrow, but whether this is connected with the formation of
 +
the nervous system, or is equivalent to the mesoblastic furrow in
 +
Insects, and connected with the formation of the mesoblast, has
 +
not been made out. Shortly after the appearance of the three
 +
pairs of appendages behind the antennae two further pairs become
 +
added, and at the same time oral and anal invaginations become
 +
formed '(fig- 173 Q. In front of the oral opening an unpaired
 +
upper lip is developed. The prse-oral part of the ventral plate
 +
develops into the bilobed procephalic lobes, the epiblast of
 +
which is mainly employed in the formation of the supra-cesophageal ganglia. The next important change which takes place is
 +
the segmentation of the body of the embryo (fig. 174 A), the
 +
most essential feature in which is the division of the mesoblast
 +
into somites. Segments are formed in order from before backwards, and soon extend to the region behind the appendages.
 +
On the appearance of segmentation the appendages commence
 +
to assume their permanent form. The two anterior pairs of
 +
post-oral appendages become jaws ; and the part of the embryo
 +
which carries them and the antennae is marked off from the
 +
trunk as the head. The three following pairs of appendages
 +
grow in length and assume a form suited for locomotion. Behind
 +
 +
 +
 +
390 CHILOGNATHA.
 +
 +
 +
 +
the three existing pairs of limbs there are developed three fresh
 +
pairs, of whicJi tJie two anterior belong to a single primitive segment. While the above changes take place in the appendages
 +
the embryo undergoes an ecdysis, which gives rise to a cuticular
 +
membrane within the single egg-membrane (chorion, Metschnikoff\ On this cuticle a tooth-like process is developed, the
 +
function of which is to assist in the hatching of the embryo
 +
(fig. 174 A).
 +
 +
In Polyxenus a cuticular membrane is present as in Strongylosoma,
 +
but it is not provided with a tooth-like process. In the same form amoeboid
 +
cells separate themselves from the blastoderm at an early period. These
 +
cells have been compared to the embryonic envelopes of Insects described
 +
below.
 +
 +
In Julus two cuticular membranes are present at the time of hatching :
 +
the inner one is very strongly developed and encloses the embryo after
 +
hatching. After leaving the chorion the embryo Julus remains connected
 +
with it by a structureless membrane which is probably the outer of the two
 +
cuticular membranes.
 +
 +
At the time when the embryo of Strongylosoma is hatched
 +
(fig. 174 B) nine post-cephalic segments appear to be present.
 +
 +
 +
 +
 +
 +
FlG. 174. TWO STAGES IN THE DEVELOPMENT OF STRONGYLOSOMA GUEKINll.
 +
 +
(After Metschnikoff.)
 +
 +
A. A seventeen days' embryo, already segmented.
 +
 +
B. A just-hatched larva.
 +
 +
Of these segments the second is apparently (from MetschnikofT's
 +
figure, 174 B) without a pair of appendages; the third and
 +
 +
 +
 +
TRACHEATA. 391
 +
 +
 +
 +
fourth are each provided with a single functional pair of limbs ;
 +
the fifth segment is provided with two pairs of rudimentary
 +
limbs, which are involuted in a single sack and not visible without preparation, and therefore not shewn in the figure. The
 +
sixth segment is provided with but a single pair of" appendages,
 +
though a second pair is subsequently developed on it 1 .
 +
 +
Julus, at the time it leaves the chorion, is imperfectly segmented, but is
 +
provided with antennas, mandibles, and maxillae, and seven pairs of limbs,
 +
of which the first three are much more developed than the remainder.
 +
Segmentation soon makes its appearance, and the head becomes distinct
 +
from the trunk, and on each of the three anterior trunk segments a single
 +
pair of limbs is very conspicuous (Metschnikoff) 2 . Each of the succeeding
 +
segments bears eventually two pairs of appendages. At the time when
 +
the inner embryonic cuticle is cast off, the larva appears to be hexapodous,
 +
like the young Strongylosoma, but there are in reality four pairs of rudimentary appendages behind the three functional pairs. The latter only
 +
appear on the surface after the first post-embryonic ecdysis. Pauropus
 +
(Lubbock) is hexapodous in a young stage. At the next moult two pairs
 +
of appendages are added, and subsequently one pair at each moult.
 +
 +
There appear to be eight post-oral segments in Julus at the
 +
time of hatching. According to Newport fresh segments are
 +
added in post-embryonic life by successive budding from a
 +
blastema between the penultimate segment and that in front of
 +
it. They arise in batches of six at the successive ecdyses, till
 +
the full number is completed. A functional, though not a real
 +
hexapodous condition, appears to be characteristic of Chilognatha
 +
generally at the time of hatching.
 +
 +
The most interesting anatomical feature of the Chilognatha
 +
is the double character of their segments, the feet (except the
 +
first three or four, or more), the circulatory, the respiratory, and
 +
the nervous systems shewing this peculiarity. Newport's and
 +
 +
1 Though the superficially hexapodous larva of Strongylosoma and other Chilognatha has a striking resemblance to some larval Insects, no real comparison is possible between them, even on the assumption that the three functional appendages of
 +
both are homologous, because Embryology clearly proves that the hexapodous Insect
 +
type has originated from an ancestor with numerous appendages by the atrophy of
 +
those appendages, and not from an hexapodous larval form prior to the development
 +
of the full number of adult appendages.
 +
 +
2 Newport states however that a pair of limbs is present on the first, second, and
 +
fourth post-oral segments, but that the third segment is apodous ; and this is undoubtedly the case in the adult.
 +
 +
 +
 +
392
 +
 +
 +
 +
CHILOPODA.
 +
 +
 +
 +
Metschnikoff's observations have not thrown as much light on
 +
the nature of the double segments as might have been hoped,
 +
but it appears probable that they have not originated from a
 +
fusion of two primitively distinct segments, but from a later
 +
imperfect division of each of the primitive segments into two,
 +
and the supply to each of the divisions of a primitive segment of
 +
a complete set of organs.
 +
 +
Chilopoda. Up to the present time the development of only
 +
one type of Chilopoda, viz. that of Geophilus, has been worked
 +
out. Most forms lay their eggs, but Scolopendra is viviparous.
 +
 +
 +
 +
 +
a u . i
 +
 +
 +
 +
FlG. 175. TWO STAGES IN THE DEVELOPMENT OF GEOPHILUS.
 +
 +
(After Metschnikoff.)
 +
 +
A. Side-view of embryo at the stage when the segments are beginning to be formed.
 +
 +
B. Later stage after the appendages have become established.
 +
 +
at. antenna.-; an.t. proctodseum.
 +
 +
The segmentation appears to resemble that in the Chilognatha,
 +
and at its close there is present a blastoderm surrounding a
 +
central mass of yolk cells. A ventral thickening of the blastoderm is soon formed. It becomes divided into numerous segments, which continue to be formed successively from the
 +
posterior unsegmented part. The antennae are the first appendages to appear, and are well developed when eighteen segments
 +
have become visible (fig. 175 A). The post-oral appendages
 +
are formed slightly later, and in order from before backwards.
 +
As the embryo grows in length, and fresh segments continue to
 +
be formed, the posterior part of it becomes bent over so as to
 +
face the ventral surface of the anterior, and it acquires an
 +
 +
 +
 +
TRACHEATA. 393
 +
 +
 +
 +
appearance something like that of many embryo Crustaceans
 +
(fig. 175 B). Between forty and fifty segments are formed while
 +
the embryo is still in the egg. The appendages long remain
 +
unjointed. The fourth post-oral appendage, which becomes the
 +
poison-claw, is early marked out by its greater size : on the
 +
third post-oral there is formed a temporary spine to open the
 +
egg membrane.
 +
 +
It does not appear, from Metschnikoff's figures of Geophilus, that any
 +
of the anterior segments are without appendages, and it is very probable
 +
that Newport is mistaken in supposing that the embryo has a segment without appendages behind that with the poison claws, which coalesces with the
 +
segment of the latter. It also appears to me rather doubtful whether the
 +
third pair of post-oral appendages, i.e. those in front of the poison-claws, can
 +
fairly be considered as forming part of the basilar plate. The basilar plate
 +
is really the segment of the poison-claws, and may fuse more or less completely with the segment in front and behind it, and the latter is sometimes
 +
without a pair of appendages (Lithobius, Scutigera).
 +
 +
Geophilus, at the time of birth, has a rounded form like that
 +
of the Chilognatha.
 +
 +
The young of Lithobius is born with only six pairs of limbs.
 +
 +
General observation on the homologies of the appendages of
 +
Myriapoda.
 +
 +
The chief difficulty in this connection is the homology of the third pair of
 +
post-oral appendages.
 +
 +
In adult Chilognatha there is present behind the mandibles a four-lobed
 +
plate, which is usually regarded as representing two pairs of appendages,
 +
viz. the first and second pairs of maxillae of Insects. Metschnikoff's observations seem however to shew that this plate represents but a single
 +
pair of appendages, which clearly corresponds with the first pair of maxillae
 +
in Insects. The pair of appendages behind this plate is ambulatory, but
 +
turned towards the head ; it is in the embryo the foremost of the three
 +
functional pairs of legs with which the larva is born. Is it equivalent to
 +
the second pair of maxillae of Insects or to the first pair of limbs of Insects?
 +
In favour of the former view is the fact (i) that in embryo Insects the
 +
second pair of maxillae sometimes resembles the limbs rather than the
 +
jaws, so that it might be supposed that in Chilognatha a primitive
 +
ambulatory condition of the third pair of appendages has been retained ;
 +
(2) that the disappearance of a pair of appendages would have to be
 +
postulated if the second alternative is adopted, and that if Insects are
 +
descended from forms related to the Myriapods it is surprising to find a
 +
pair of appendages always present in the former, absent in the latter.
 +
 +
 +
 +
394
 +
 +
 +
 +
MYRIAPODA.
 +
 +
 +
 +
The arguments which can be urged for the opposite view do not appear
 +
to me to have much weight, so that the homology of the appendages in
 +
question with the second pair of maxillae may be provisionally assumed.
 +
 +
The third pair of post-oral appendages of the Chilopoda may probably
 +
also be assumed to be equivalent to the second pair of maxillae, though they
 +
are limb-like and not connected with the head. The subjoined table shews
 +
the probable homologies of the appendages.
 +
 +
 +
 +
 +
 +
CHILOGNATHA(Strongylo
 +
so ma at time of birth).
 +
 +
 +
CHILOPODA (Scolopendra
 +
adult).
 +
 +
 +
Pre-oral region.
 +
 +
 +
Antennae.
 +
 +
 +
Antennas.
 +
 +
 +
ist Post-oral segment.
 +
 +
 +
Mandibles.
 +
 +
 +
Mandibles.
 +
 +
 +
2nd ,, ,,
 +
 +
 +
Maxillae i. (Four-lobed
 +
plate in adult, but a simple pair of appendages
 +
in embryo).
 +
 +
 +
Maxillie i.
 +
(Palp and bilobed median
 +
process).
 +
 +
 +
3rd
 +
(probably equivalent to
 +
segment bearing 2nd pair
 +
of maxillae in Insects).
 +
 +
 +
ist pair of ambulatory
 +
limbs.
 +
 +
 +
Limb-like appendages with
 +
basal parts in contact.
 +
 +
 +
4th ,, ,,
 +
 +
 +
(?) Apodous.
 +
 +
 +
Poison claws.
 +
 +
 +
5th
 +
 +
 +
2nd pair of ambulatory
 +
limbs.
 +
 +
 +
ist pair of ambulatory
 +
limbs.
 +
 +
 +
6th
 +
 +
 +
3rd ,,
 +
 +
 +
2nd ,,
 +
 +
 +
7th
 +
 +
 +
4th and sth
 +
(rudimentary. )
 +
 +
 +
3rd
 +
 +
8th ,, ,,
 +
 +
 +
6th
 +
(the 7th pair is developed
 +
in this segment later).
 +
 +
 +
4th
 +
 +
 +
9 th
 +
 +
 +
Apodous.
 +
 +
 +
5th
 +
 +
 +
loth ,,
 +
 +
 +
,, (last segment in
 +
embryo).
 +
 +
 +
6th
 +
 +
 +
 +
The germinal layers and formation of organs.
 +
 +
The development of the organs of the Myriapoda, and the origin of the
 +
germinal layers, are very imperfectly known : Myriapoda appear however
 +
to be closely similar to Insects in this part of their development, and the
 +
general question of the layers will be treated more fully in connection with
 +
that group.
 +
 +
The greater part of the blastoderm gives rise to the epiblast, which
 +
furnishes the skin, nervous system, tracheal system, and the stomodacum
 +
and proctodaeum.
 +
 +
 +
 +
TRACHEATA. 395
 +
 +
 +
 +
The mesoblast arises in connection with the ventral thickening of the
 +
blastoderm, but the details of its formation are not known. Metschnikoff
 +
describes a longitudinal furrow which appears very early in Strongylosoma,
 +
which is perhaps equivalent to the mesoblastic furrows of Insects, and so
 +
connected with the formation of the mesoblast.
 +
 +
The mesoblast is divided up into a series of protovertebra-like bodies
 +
the mesoblastic somites the cavities of which become the body cavity and
 +
the walls the muscles and probably the heart. They are (Metschnikoff)
 +
prolonged into the legs, though the prolongations become subsequently
 +
segmented off from the main masses. The splanchnic mesoblast is,
 +
according to Metschnikoff, formed independently of the somites, but this
 +
point requires further observation.
 +
 +
The origin of the hypoblast remains uncertain, but it appears probable
 +
that it originates, in a large measure at least, from the yolk segments. In
 +
the Chilognatha the mesenteron is formed in the interior of the yolk segments, so that those yolk segments which are not employed in the formation
 +
of the alimentary canal lie freely in the body cavity. In the relation of
 +
the yolk segments to the alimentary canal the Chilopoda present a strong
 +
contrast to the Chilognatha, in that the greater part of the yolk lies
 +
within their mesenteron. The mesenteron is at first a closed sack, but is
 +
eventually placed in communication with the stomodaeum and the proctodasum. The Malpighian bodies arise as outgrowths from the blind extremity of the latter.
 +
 +
BIBLIOGRAPHY.
 +
 +
(397) G. N e wp or t. " On the Organs of Reproduction and Development of the
 +
Myriapoda." Philosophical Transactions, 1841.
 +
 +
(398) E. Metschnikoff. ' ' Embryologie der doppeltflissigen Myriapoden (Chilognatha)." Zeit.f. wiss. Zool., Vol. xxiv. 1874.
 +
 +
(399) ' ' Embryologisches iiber Geophilus." Zeit. f. wiss. ZooL y Vol. xxv.
 +
 +
1875
 +
(400) Anton Stecker. "Die Anlage d. Keimblatter bei den Diplopoden."
 +
Archivf. mik. Anatomie, Bd. xiv. 1877.
 +
 +
INSECTA 1 .
 +
 +
The formation of the embryonic layers in Insects has not
 +
been followed out in detail in a large number of types ; but, as
 +
 +
1 The following classification of the Insecta is employed in this chapter,
 +
((i) Collembola.
 +
 +
I. Aptera. | (a) Thysanura .
 +
 +
!(i) Orthoptera genuina (Blatta, Locusta, etc.).
 +
(2) pseudoneuroptera (Termes, Ephemera,
 +
 +
Libellula).
 +
 +
!(i) Hemiptera heteroptera (Cimex, Notonecta, etc.).
 +
(2) ,, homoptera (Aphis, Cicada, etc.).
 +
 +
(3) ,, parasita (Pediculus, etc.).
 +
 +
 +
 +
396
 +
 +
 +
 +
INSECTA.
 +
 +
 +
 +
in so many other instances, some of the most complete histories
 +
we have are due to Kowalevsky (No. 416). The development
 +
 +
 +
 +
 +
FiG. 176. FOUR EMBRYOS OF llYDROPHlLUS P1CEUS VIEWED FROM THE
 +
 +
VENTRAL SURFACE. (After Kowalevsky.)
 +
The upper end is the anterior, gg. germinal groove; am. amnion.
 +
 +
of Hydrophilus has been worked out by him more fully than
 +
that of any other form, and will serve as a type for comparison
 +
with other forms.
 +
 +
The segmentation has not been studied, but no doubt belongs
 +
to the centrolecithal type (vide pp. no 120). At its close
 +
there is an uniform layer of cells enclosing a central mass of
 +
yolk. These cells, in the earliest observed stage, were flat on
 +
the dorsal, but columnar on part of the ventral surface of the
 +
egg, where they form a thickening which will be called the ventral plate. At the posterior part of the ventral plate two folds,
 +
with a furrow between them, make their appearance. They form
 +
a structure which may be spoken of as the germinal groove (fig.
 +
 +
!(i) Diptera genuina (Musca, Tipula, etc.).
 +
(2) aphaniptera (Pulex, etc.).
 +
(3) ,, pupipara (Braula, etc.).
 +
 +
v .. ( (i) Neuroptera planipennia (Myrniclcon, etc.)
 +
TOptera. j (a) ^ trichoptera (Phryganea, etc.).
 +
 +
VI. Coleoptera.
 +
VII. Lepidoptera.
 +
 +
(i) Hymenoptera aculeata (Apis, Formica, etc.).
 +
(a) ,, entomophaga (Ichneumon, Platy
 +
gaster, etc).
 +
(3) ,, phytophaga ( Tenthredo, Sirex, etc.).
 +
 +
 +
 +
VIII. Hymenoptera.
 +
 +
 +
 +
TRACHEATA.
 +
 +
 +
 +
397
 +
 +
 +
 +
 +
y*
 +
 +
 +
 +
FlG. 177. TWO TRANSVERSE SECTIONS THROUGH
 +
 +
EMBRYOS OF HvDROPHiLUS piCEUS. (After Kowalevsky.)
 +
 +
A. Section through an embryo of the stage represented in fig. 176 B, at the point where the two
 +
germinal folds most approximate.
 +
 +
B. Section through an embryo somewhat later
 +
than the stage fig. 176 D, through the anterior region
 +
where the amnion has not completely closed over the
 +
embryo.
 +
 +
 +
 +
). The cells
 +
which form the floor
 +
of the groove are far
 +
more columnar than
 +
those of other parts
 +
of the blastoderm (fig.
 +
177 A). The two
 +
folds on each side of
 +
it gradually approach
 +
each other. They do
 +
so at first behind, and
 +
then in the middle;
 +
from the latter point
 +
the approximation
 +
gradually extends
 +
backwards and forwards (fig. 176 B and
 +
C). In the middle
 +
and hinder parts of
 +
the ventral plate the
 +
groove becomes, by
 +
the coalescence of the folds, converted into a canal (fig. 178 A,
 +
gg), the central cavity of which soon disappears, while at the
 +
same time the cells of the wall undergo division, become more
 +
rounded, and form a definite layer (me} the mesoblast beneath
 +
the columnar cells of the surface. Anteriorly the process is
 +
slightly different, though it leads to the similar formation of
 +
mesoblast (fig. 177 B). The flat floor of the groove becomes in
 +
front bodily converted into the mesoblast, but the groove itself
 +
is never converted into a canal. The two folds simply meet
 +
above, and form a continuous superficial layer.
 +
 +
During the later stages of the process last described remarkable structures, eminently characteristic of the Insecta, have
 +
made their first appearance. These structures are certain
 +
embryonic membranes or coverings, which present in their mode
 +
of formation and arrangement a startling similarity to the true
 +
and false amnion of the Vertebrata. They appear as a double
 +
fold of the blastoderm round the edge of the germinal area,
 +
which spreads over the ventral plate, from behind forwards, in a
 +
 +
 +
 +
gg. germinal groove ;
 +
nion ; yk. yolk.
 +
 +
 +
 +
me. mesoblast ; am. am
 +
 +
 +
INSECT A.
 +
 +
 +
 +
 +
general way in the same
 +
manner as the amnion in,
 +
for instance, the chick.
 +
The folds at their origin
 +
are shewn in surface view
 +
in fig. 176 D, am, and in
 +
section in fig. 177 B, am.
 +
The folds eventually
 +
meet, coalesce (fig. 178,
 +
am) and give rise to two
 +
membranes covering the
 +
ventral plate, viz. an
 +
inner one, which is continuous with the edge of
 +
the ventral plate ; and
 +
an outer, continuous with
 +
the remainder of the
 +
blastoderm. The vertebrate nomenclature may
 +
be conveniently employed for these membranes.
 +
The inner limb of the
 +
fold will therefore be spoken of as the amnion, and the outer
 +
one, including the dorsal part of the blastoderm, as the
 +
serous envelope 1 . A slight consideration of the mode of
 +
formation of the membranes, or an inspection of the figures
 +
illustrating their formation, makes it at once clear that the yolk
 +
can pass in freely between the amnion and serous envelope (vide
 +
fig. 181). At the hind end of the embryo this actually takes
 +
place, so that the ventral plate covered by the amnion appears to
 +
become completely imbedded in the yolk: elsewhere the two
 +
membranes are in contact. At first (fig. 176) the ventral plate
 +
occupies but a small portion of the ventral surface of the egg, but
 +
during the changes above described it extends over the whole
 +
ventral surface, and even slightly on the dorsal surface both in
 +
front and behind. It becomes at the same time (fig. 179) divided
 +
 +
 +
 +
FIG. 178. SECTIONS THROUGH TWO EMBRYOS
 +
OF HYDROPHILUS PICEUS. (After Kowalevsky.)
 +
 +
A. Section through the posterior part of the
 +
embryo fig. 1 76 D, shewing the completely closed
 +
amnion and the germinal groove.
 +
 +
B. Section through an older embryo in which
 +
the mesoblast has grown out into a continuous
 +
plate beneath the epiblast.
 +
 +
gg. germinal groove ; am. amnion ; yk. yolk ;
 +
cp. epiblast.
 +
 +
 +
 +
1 The reverse nomenclature to this is rather inconveniently employed by Metschnikoff.
 +
 +
 +
 +
TRACHEATA.
 +
 +
 +
 +
399
 +
 +
 +
 +
 +
FIG. 179. EMBRYO OF
 +
HYDROPHILUS PICEUS
 +
 +
VIEWED FROM THE VEN
 +
TRAL SURFACE. (After
 +
Kowalevsky.)
 +
 +
pc.L procephalic lobe.
 +
 +
 +
 +
by a series of transverse lines into segments, which increase in
 +
number and finally amount in all to seventeen, not including the most anterior section,
 +
which gives off as lateral outgrowths the
 +
two procephalic lobes (pc.l). The changes
 +
so far described are included within what
 +
Kowalevsky calls his first embryonic period;
 +
at its close the parts contained within the
 +
chorion have the arrangement shewn in fig.
 +
178 B. The whole of the body of the
 +
embryo is formed from the ventral plate,
 +
and no part from the amnion or serous
 +
envelope.
 +
 +
The general history of the succeeding
 +
stages may be briefly told.
 +
 +
The appendages appear as very small
 +
rudiments at the close of the last stage, but
 +
soon become much more prominent (fig.
 +
1 80 A). They are formed as outgrowths of both layers, and
 +
arise nearly simultaneously. There
 +
are in all eight pairs of appendages.
 +
The anterior or antennae (at) spring
 +
from the procephalic lobes, and
 +
the succeeding appendages from
 +
the segments following. The last
 +
pair of embryonic appendages,
 +
which disappears very early, is
 +
formed behind the third pair of
 +
the future thoracic limbs. Paired
 +
epiblastic involutions, shewn as pits
 +
in the posterior segments in fig.
 +
1 80 A, give rise to the tracheae;
 +
and the nervous system is formed
 +
as two lateral epiblastic thickenings, one on each side of the midventral line. These eventually become split off from the skin ; while
 +
between them there passes in a
 +
median invagination of the skin
 +
 +
 +
 +
 +
FlG. 1 80. TWO STAGES IN THE
 +
DEVELOPMENT OF HYDROPHILUS
 +
 +
PICEUS. (From Gegenbaur, after
 +
Kowalevsky.)
 +
 +
Is. labrum ; at. antenna ; md.
 +
 +
 +
 +
400 INSECTA.
 +
 +
 +
 +
(fig. 189 C). The two nervous strands are continuous in front
 +
with the supra-oesophageal ganglia, which are formed of the
 +
epiblast of the procephalic lobes. These plates gradually grow
 +
round the dorsal side of the embryo, and there is formed
 +
immediately behind them an oral invagination, in front of which
 +
there appears an upper lip (fig. 180, Is). A proctodaeum is formed
 +
at the hind end of the body slightly later than the stomodaeum.
 +
The mesoblast cells become divided into two bands, one on
 +
each side of the middle line (fig. 189 A), and split into
 +
splanchnic and somatic layers. The central yolk mass at about
 +
the stage represented in fig. 179 begins to break up into
 +
yolk spheres. The hypoblast is formed first on the ventral
 +
side at the junction of the mesoblast and the yolk, and
 +
gradually extends and forms a complete sack-like mesenteron,
 +
enveloping the yolk (fig. 185 al). The amnion and serous
 +
membrane retain their primitive constitution for some time, but
 +
gradually become thinner on the ventral surface, where a rupture
 +
appears eventually to take place. The greater part of them
 +
disappears, but in the closure of the dorsal parietes the serous
 +
envelope plays a peculiar part, which is not yet understood. It
 +
is described on p. 404. The heart is formed from the mesoblastic layers, where they meet in the middle dorsal line (fig. 185 C,
 +
hi]. The somatic mesoblast gives rise to the muscles and
 +
connective tissue, and the splanchnic mesoblast to the muscular
 +
part of the wall of the alimentary tract, which accompanies the
 +
hypoblast in its growth round the yolk. The proctodaeum
 +
forms the rectum and Malpighian bodies 1 , and the stomodseum
 +
the oesophagus and proventriculus. The two epiblastic sections
 +
of the alimentary tract are eventually placed in communication
 +
with the mesenteron.
 +
 +
The development of Hydrophilus is a fair type of that of
 +
Insects generally, but it is necessary to follow with somewhat
 +
greater detail the comparative history of the various parts which
 +
have been briefly described for this type.
 +
 +
TJte embryonic membranes and the formation of the layers.
 +
 +
All Insects have at the close of segmentation a blastoderm
 +
formed of a single row of cells enclosing a central yolk mass,
 +
 +
1 This has not been shewn in the case of Hydrophilus,
 +
 +
 +
 +
TRACHEATA.
 +
 +
 +
 +
401
 +
 +
 +
 +
which usually contains nuclei, and in the Poduridae is divided up
 +
in the ordinary segmentation into distinct yolk cells. The first
 +
definite structure formed is a thickening of the blastoderm,
 +
which forms a ventral plate.
 +
 +
The ventral plate is very differently situated in relation to the yolk in
 +
different types. In most Diptera, Hymenoptera and (?) Neuroptera (Phryganea) it forms from the first a thickening extending over nearly the
 +
whole ventral surface of the ovum, and in many cases extends in its subsequent growth not only over the whole ventral surface, but over a considerable part of the apparent dorsal surface as well (Chironomus, Simulia,
 +
Gryllotalpa, etc.). In Coleoptera, so far as is known, it commences as a less
 +
extended thickening either of the central part (Donacia) or posterior part
 +
(Hydrophilus) of the ventral surface, and gradually grows in both directions,
 +
passing over to the dorsal surface behind.
 +
 +
Embryonic membranes. In the majority of Insects there
 +
are developed enveloping membranes like those of Hydrophilus.
 +
 +
The typical mode of formation of these membranes is represented diagrammatically in fig. 181 A and B. A fold of the
 +
blastoderm arises round the edge of the ventral plate. This
 +
fold, like the amniotic fold of the
 +
higher Vertebrata,
 +
is formed of two
 +
limbs, an outer,
 +
the serous membrane (se), and an
 +
inner, the true amnion (am). Both
 +
limbs extend so
 +
as to cover over
 +
the ventral plate,
 +
and finally meet
 +
and coalesce, so
 +
thatadouble membrane is present
 +
over the ventral
 +
plate. At the same
 +
time (fig. 181 B)
 +
the point where the fold originates is carried dorsalwards by the
 +
B. II. 26
 +
 +
 +
 +
Sf
 +
 +
 +
 +
 +
FIG. 181. DIAGRAMMATIC LONGITUDINAL SECTIONS
 +
OF AN INSECT EMBRYO AT TWO STAGES TO SHEW THE
 +
 +
DEVELOPMENT OF THE EMBRYONIC ENVELOPES.
 +
 +
In A the amniotic folds have not quite met so as to
 +
cover the ventral plate. The yolk is represented as divided
 +
into yolk cells. In B the sides of the ventral plate have
 +
extended so as nearly to complete the dorsal integument.
 +
The mesenteron is represented as a closed sack filled with
 +
yolk cells, am. amnion; se. serous envelope; v.p. ventral plate ; d. i. dorsal integument ; me. mesenteron ; st.
 +
stomodaeum ; an i. proctodaeum.
 +
 +
 +
 +
4O2 INSECTA.
 +
 +
 +
 +
dorsal extension of the edges of the ventral plate, which give
 +
rise to the dorsal integument (d.i). This process continues
 +
till the whole dorsal surface is covered by the integument.
 +
The amnion then separates from the dorsal integument, and the
 +
embryo becomes enveloped in two membranes an inner, the
 +
amnion, and an outer, the serous membrane. In fig. 181 B the
 +
embryo is represented at the stage immediately preceding the
 +
closure of the dorsal surface.
 +
 +
By the time that these changes are effected, the serous
 +
membrane and amnion are both very thin and not easily
 +
separable. The amnion appears to be usually absorbed before
 +
hatching; but in hatching both membranes, if present, are either
 +
absorbed, or else ruptured and thrown off.
 +
 +
The above mode of development of the embryonic membranes has been
 +
especially established by the researches of Kowalevsky (No. 416) and Graber
 +
(No. 412) for various Hymenoptera (Apis), Diptera (Chironomus\ Lepidoptera and Coleoptera (Melolontha, Lino).
 +
 +
Considerable variations in the development of the enveloping membranes
 +
are known.
 +
 +
When the fold which gives rise to the membranes is first formed, there
 +
is, as is obvious in fig. 181 A, a perfectly free passage by which the yolk can
 +
pass in between the amnion and serous membrane. Such a passage of the
 +
yolk between the two membranes takes place posteriorly in Hydrophilus and
 +
Donacia: in Lepidoptera the yolk passes in everywhere, so that in this form
 +
the ventral plate becomes first of all imbedded in the yolk, and finally, on the
 +
completion of the dorsal integument, the embryo is enclosed in a complete
 +
envelope of yolk contained between the amnion and the serous membrane.
 +
During the formation of the dorsal integument the external yolk sack communicates by a dorsally situated umbilical canal with the yolk cavity within
 +
the body. On the rupture of the amnion the embryo is nourished at the
 +
expense of the yolk contained in the external yolk sack.
 +
 +
In the Hemiptera and the Libellulidae the ventral plate also becomes
 +
imbedded in the yolk, but in a somewhat different fashion to the Lepidoptera, which more resembles on an exaggerated scale what takes place in
 +
Hydrophilus.
 +
 +
In the Libellulidas (Calopteryx) there is first of all formed (Brandt, No.
 +
403) a small ventral and posterior thickening of the blastoderm (fig. 182 A).
 +
The hinder part of this becomes infolded into the yolk as a projection (fig.
 +
182 B), which consists of two laminae, an anterior and a posterior, continuous
 +
at the apex of the invagination. The whole structure, which is completely
 +
imbedded within the yolk, rapidly grows in length, and turns towards
 +
the front end of the egg (fig. 182 C). Its anterior lamina remains thick and
 +
gives rise to the ventral plate (ps), the posterior (am) on the other hand
 +
 +
 +
 +
TRACHEATA.
 +
 +
 +
 +
403
 +
 +
 +
 +
 +
becomes very thin, and
 +
forms a covering corresponding with the amnion
 +
of the more ordinary types.
 +
The remainder of the blastoderm covering the yolk
 +
(se) forms the homologue
 +
of the serous membrane
 +
of other types. The ventral surface of the ventral
 +
plate is turned towards
 +
the dorsal side (retaining
 +
the same nomenclature as
 +
in ordinary cases) of the
 +
egg, and the cephalic
 +
extremity is situated at
 +
the point of origin of the
 +
infolding.
 +
 +
The further history is
 +
however somewhat peculiar. The amnion is at first
 +
(fig. 182 C) continuous with
 +
the serous envelope on the
 +
posterior side only, so that
 +
the serous envelope does
 +
not form a continuous sack,
 +
but has an opening close
 +
to the head of the embryo.
 +
In the Hemiptera parasita this opening (Melnikow, No. 422) remains permanent, and the embryo, after it has reached a certain stage of development,
 +
becomes everted through it, while the yolk, enclosed in the continuous membrane formed by the amnion and serous envelope, forms a yolk sack on the
 +
dorsal surface. In the Libellulidae however and most Hemiptera, a fusion of
 +
the two limbs of the serous membrane takes place in the usual way, so as to
 +
convert it into a completely closed sack (fig. 183 A). After the formation of
 +
the appendages a fusion takes place between the amnion and serous envelope over a small area close to the head of the embryo. In the middle of
 +
this area a rupture is then effected, and the head of the embryo followed by
 +
the body is gradually pushed through the opening (fig. 183 B and C). The
 +
embryo becomes in the process completely rotated, and carried into a
 +
position in the egg-shell identical with that of the embryos of other orders of
 +
Insects (fig. 183 C).
 +
 +
Owing to the rupture of the embryonic envelopes taking place at the
 +
point where they are fused into one, the yolk does not escape in the above
 +
process, but is carried into a kind of yolk sack, on the dorsal surface of the
 +
embryo, formed of the remains of the amnion and serous envelope. The
 +
 +
262
 +
 +
 +
 +
FIG. 182. THREE STAGES IN THE DEVELOPMENT
 +
 +
OF THE EMBRYO OF CALOPTERYX. (After Brandt.)
 +
 +
The embryo is represented in the egg-shell.
 +
 +
A. Embryo with ventral plate.
 +
 +
B. Commencing involution of ventral plate.
 +
 +
C. Involution of ventral plate completed.
 +
 +
ps. vefitral plate; g. edge of ventral plate; am.
 +
amnion ; se- serous envelope.
 +
 +
 +
 +
404
 +
 +
 +
 +
INSECTA.
 +
 +
 +
 +
walls of the yolk sack either
 +
assist in forming the dorsal
 +
parietes of the body, or are
 +
more probably enclosed
 +
within the body by the
 +
growth of the dorsal parietes from the edge of the
 +
ventral plate.
 +
 +
In Hydrophilus and
 +
apparently in the Phryganidae also, there are certain remarkable peculiarities in the closure of the
 +
dorsal surface. The fullest
 +
observations on the subject
 +
have been made by Kowalevsky (No. 416), but Dohrn
 +
(No. 408) has with some
 +
probability thrown doubts
 +
on Kowalevsky's interpretations. According to Dohrn
 +
the part of the serous envelope which covers the dorsal surface becomes thickened, and gives rise to a
 +
peculiar dorsal plate which
 +
is shewn in surface view in
 +
ventral parts of the amnion
 +
and serous membrane have
 +
either been ruptured or
 +
have disappeared. While
 +
the dorsal plate is being
 +
formed, the mesoblast, and
 +
somewhat later the lateral
 +
parts of the epiblast of the
 +
ventral plate gradually
 +
grow towards the dorsal
 +
side and enclose the dorsal
 +
plate, the wall of which in
 +
the process appears to be
 +
folded over so as first of
 +
all to form a groove and
 +
finally a canal. The stages
 +
in this growth are shewn
 +
from the surface in fig. 184
 +
B and C and in section in
 +
 +
 +
 +
 +
FlG. 183. THREE STAGES IN THE DEVELOPMENT
 +
 +
OF CALOPTERYX. (After Brandt.)
 +
 +
The embryo is represented in the egg-shell; B.
 +
and C. shew the inversion of the embryo.
 +
 +
sf. serous envelope ; am. amnion ; ab. abdomen ;
 +
v. anterior end of head ; at. antennae ; md. mandible ;
 +
mx l . maxilla i ; mx*. maxilla 2 ; p 1 ^. three pairs
 +
of legs; oe. oesophagus.
 +
 +
fig. 184 A, doi and in section in fig. 185 A, do. The
 +
 +
 +
 +
 +
FIG. 184. THREE LARVAL STAGES OF HYDROPHILUS FROM THE DORSAL SIDE, SHEWING THE
 +
GRADUAL CLOSING IN OF THE DORSAL REGION WITH
 +
THE FORMATION < >! THK I'l.CULIAR DORSAL ORGAN
 +
 +
do. (After Kowalevsky.)
 +
 +
do. dorsal organ ; at. antennae.
 +
 +
 +
 +
TRACHEATA.
 +
 +
 +
 +
405
 +
 +
 +
 +
fig. 185 B, do. The canal is buried on the dorsal part of the yolk, but for
 +
some time remains open by a round aperture in front (fig. 184 C). The
 +
whole structure is known as the dorsal canal. It appears to atrophy without
 +
leaving a trace. The heart when formed lies immediately dorsal to it 1 .
 +
 +
 +
 +
 +
A.
 +
B.
 +
C.
 +
 +
 +
 +
vn
 +
 +
 +
 +
FIG. 185. THREE TRANSVERSE SECTIONS THROUGH ADVANCED
 +
 +
EMBRYOS OF HYDROPHILUS.
 +
 +
Section through the posterior part of the body of the same age as fig. 184 A.
 +
Section through the embryo of the same age as fig. 184 C.
 +
Section through a still older embryo.
 +
do. dorsal plate ; vn. ventral nerve cord ; al. mesenteron ; ht. heart.
 +
The large spaces at the sides are parts of the body cavity.
 +
 +
In the Poduridas the embryonic membranes appear to be at any rate
 +
imperfect. Metschnikoff states in his paper on Geophilus that in some ants
 +
no true embryonic membranes are found, but merely scattered cells which
 +
take their place. In the Ichneumonidas the existence of two embryonic
 +
membranes is very doubtful.
 +
 +
Formation of the embryonic layers. The formation of the
 +
layers has been studied in sections by Kowalevsky (No. 416),
 +
 +
1 According to Kowalevsky the history of the dorsal plate is somewhat different.
 +
He believes that on the absorption of the amnion the ventral plate unites with the
 +
serous membrane, and that the latter directly gives rise to the dorsal integument,
 +
while the thickened part of it becomes involuted to form the dorsal tube already
 +
described.
 +
 +
 +
 +
406 INSECTA.
 +
 +
 +
 +
Hatschek (No. 414), and Graber (No. 412), etc. From their
 +
researches it would appear that the formation of the mesoblast
 +
always takes place in a manner closely resembling that in
 +
Hydrophilus. The essential features of the process (figs. 177
 +
and 178) appear to be that a groove is formed along the median
 +
line of the ventral plate, and that the sides of this groove either
 +
(i) simply close over like the walls of the medullary groove in
 +
Vertebrates, and so convert the groove into a tube, which soon
 +
becomes solid and forms a mass or plate of cells internal to the
 +
epiblast ; or (2) that the cells on each side of the groove grow
 +
over it and meet in the middle line, forming a layer external
 +
to the cells which lined the groove. The former of these
 +
processes is the most usual ; and in the Muscidae the dimensions
 +
of the groove are very considerable (Graber, No. 411). In both
 +
cases the process is fundamentally the same, and causes the
 +
ventral plate to become divided into two layers 1 . The external
 +
layer or epiblast is an uniform sheet forming the main part of
 +
the ventral plate (fig. 178 B, ep). It is continuous at its edge
 +
with the amnion. The inner layer or mesoblast constitutes an
 +
independent plate of cells internal to the epiblast (fig. 178 B, me).
 +
The mesoblast soon becomes divided into two lateral bands.
 +
 +
The origin of the hypoblast is still in dispute. It will be
 +
remembered (vide pp. 1 14 and 1 16) that after the segmentation a
 +
number of nuclei remain in the yolk ; and that eventually a
 +
secondary segmentation of the yolk takes place around these
 +
nuclei, and gives rise to a mass of yolk cells, which fill up the
 +
interior of the embryo. These cells are diagrammatically shewn
 +
in figs. 181 and 189, and it is probable that they constitute the
 +
true hypoblast. Their further history is given below.
 +
 +
Formation of the organs and their relation to the germinal
 +
 +
layers.
 +
 +
The segments and appendages. One of the earliest
 +
phenomena in the development is the appearance of transverse
 +
lines indicating segmentation (fig. 186). The transverse lines
 +
are apparently caused by shallow superficial grooves, and also in
 +
 +
1 Tichomiroff (No. 420) denies the existence of a true invagination to form the
 +
mesoblast, and also asserts that a separation of mesoblast cells from the epiblast can
 +
take place at other parts besides the median ventral line.
 +
 +
 +
 +
TRACHEATA.
 +
 +
 +
 +
407
 +
 +
 +
 +
many cases by the division of the mesoblastic bands into
 +
separate somites. The most anterior line marks off a prae-oral
 +
segment, which soon sends out two lateral wings the procephalic
 +
lobes. The remaining segments are at first fairly uniform.
 +
Their number does not, however, appear to be very constant.
 +
So far as is known they never exceed seventeen, and this
 +
number is probably the typical one (figs. 186 and 187).
 +
 +
In Diptera the number appears to be usually fifteen though it may be
 +
only fourteen. In Lepidoptera and in Apis there appear to be sixteen
 +
segments. These and other variations affect only the number of the segments
 +
which form the abdomen of the adult.
 +
 +
The appendages arise as paired pouchlike outgrowths of the epiblast and mesoblast ; and their number and the order of
 +
their appearance are subject to considerable
 +
variation, the meaning of which is not yet
 +
clear. As a rule they arise subsequently to
 +
the segmentation of the parts of the body
 +
to which they belong. There is always
 +
formed one pair of appendages which spring
 +
from the lateral lobes of the procephalic
 +
region, or from the boundary line between
 +
these and the median ventral part of this
 +
region. These appendages are the antennae.
 +
They have in the embryo a distinctly ventral position as compared to that which
 +
they have in the adult.
 +
 +
In the median ventral part of the procephalic region there arises the labrum (fig. 187, Is}. It is formed
 +
by the coalescence of a pair of prominences very similar to true
 +
appendages, though it is probable that they have not this
 +
value 1 .
 +
 +
1 If these structures are equivalent to appendages, they may correspond to one of
 +
the pairs of antennae of Crustacea. From a figure by Fritz Miiller of the larva of
 +
Calotermes (Jenaische Zeit. Vol. XI. pi. n, fig. 12) it would appear that they lie in
 +
front of the true antennae, and would therefore on the above hypothesis correspond to
 +
the first pair of antennae of Crustacea. Biitschli (No. 405) describes in the Bee a pair
 +
of prominences immediately in front of the mandibles which eventually unite to form
 +
a kind of underlip ; they in some ways resemble true appendages.
 +
 +
 +
 +
 +
FIG. 1 86. EMBRYO
 +
OF HYDROPHILUS PI
 +
CEUS VIEWED FROM THE
 +
VENTRAL SURFACE.
 +
 +
(After Kowalevsky.)
 +
pc. I. procephalic lobe.
 +
 +
 +
 +
408
 +
 +
 +
 +
INSECTA.
 +
 +
 +
 +
The antennae themselves can hardly be considered to have
 +
the same morphological value as the succeeding appendages.
 +
They are rather equivalent to paired processes of the prae-oral
 +
lobes of the Chaetopoda.
 +
 +
From the first three post-oral segments there grow out the
 +
mandibles and two pairs of maxillae, and from the three following
 +
segments the three pairs of thoracic appendages. In many
 +
Insects (cf. Hydrophilus) a certain .number of appendages of the
 +
same nature as the anterior ones are visible in the embryo on
 +
the abdominal segments, a fact which shews that Insects are
 +
descended from ancestors with more than three pairs of ambulatory appendages.
 +
 +
In Apis according to Biitschli (No. 405) all the abdominal segments are
 +
provided with appendages, which always
 +
remain in a very rudimentary condition.
 +
All trace of them as well as of the thoracic
 +
appendages is lost by the time the embryo
 +
is hatched. In the phytophagous Hymenoptera the larva is provided with
 +
9 ii pairs of legs.
 +
 +
In the embryo of Lepidoptera there
 +
would appear from Kowalevsky's figures
 +
to be rudiments of ten pairs of post-thoracic appendages. In the caterpillar of
 +
this group there are at the maximum five
 +
pairs of such rudimentary feet, viz. a pair
 +
on the 3rd, 4th, 5th, and 6th, and on the
 +
last abdominal segment. The embryos
 +
of Hydrophilus (fig. 187), Mantis, etc. are
 +
also provided with additional appendages.
 +
In various Thysanura small prominences
 +
are present on more or fewer of the abdominal segments (fig. 192), which may
 +
probably be regarded as rudimentary
 +
feet.
 +
 +
Whether all or any of the appendages
 +
of various kinds connected with the
 +
hindermost segments belong to the same
 +
category as the legs is very doubtful. Their usual absence in the embryo or
 +
in any case their late appearance appears to me against so regarding them ;
 +
but Biitschli is of opinion that in the Bee the parts of the sting are related
 +
genetically to the appendages of the penultimate and antepenultimate abdominal segments, and this view is to some extent supported by more recent
 +
 +
 +
 +
 +
FlG. 187. TWO STAGES IN THE
 +
DEVELOPMENT OF HYDROPHILUS
 +
 +
PICEUS. (From Gegenbaur, after
 +
Kowalevsky. )
 +
 +
Is. labrum; at. antenna; tnd.
 +
mandible; nix. maxilla I.; li. maxilla II.; //>"/" feet; a. anus.
 +
 +
 +
 +
TRACHEATA.
 +
 +
 +
 +
409
 +
 +
 +
 +
observations (Kraepelin, etc.), and if it holds true for the Bee must be regarded
 +
as correct for other cases also.
 +
 +
As to the order of the appearance of the appendages observations are as
 +
yet too scanty to form any complete scheme. In many cases all the appendages appear approximately at the same moment, e.g. Hydrophilus, but
 +
whether this holds good for all Coleoptera is by no means certain. In Apis
 +
the appendages are stated by Biitschli to arise simultaneously, but according
 +
to Kowalevsky the two mouth appendages first appear, then the antennae,
 +
and still later the thoracic appendages. In the Diptera the mouth appendages are first formed, and either simultaneously with these, or slightly later,
 +
the antennae. In the Hemiptera and Libellulidae the thoracic appendages
 +
are the first to be formed, and the second pair of maxillae makes its appearance before the other cephalic appendages.
 +
 +
The history of the changes in the embryonic appendages during the
 +
attainment of the 'adult con- .
 +
 +
dition is beyond the scope
 +
of this treatise, but it may
 +
be noted that the second
 +
pair of maxillae are relatively very large in the
 +
embryo, and not infrequently (Libellula, etc.)
 +
have more resemblance to
 +
the ambulatory than to the
 +
masticatory appendages.
 +
 +
The exact nature of the
 +
wings and their relation to
 +
the other segments is still
 +
very obscure. They appear as dorsal leaf-like appendages on the 2nd and
 +
3rd thoracic segments, and
 +
are in many respects similar to the tracheal gills
 +
of the larvae of Ephemeridae and Phryganidae (fig.
 +
1 88 A), of which they are
 +
supposed by Gegenbaur
 +
and Lubbock to be modifications. The undoubtedly
 +
secondary character of the
 +
closed tracheal system of
 +
larvae with tracheal gills
 +
tells against this view.
 +
Fritz Miiller finds in the
 +
larvae of Calotermes ru
 +
 +
 +
 +
FIG. 188. FIGURES ILLUSTRATING AQUATIC RESPIRATION IN INSECTS. (After Gegenbaur.)
 +
 +
A. Hinder portion of the body of Ephemera
 +
vulgata. a. longitudinal tracheal trunks; b. alimentary canal ; c. tracheal gills.
 +
 +
B. Larva of ^Eschna grandis. a. superior longitudinal tracheal trunks ; b. their anterior end ; c. portion branching on proctodaeum ; o. eyes.
 +
 +
C. Alimentary canal of the same larva from the
 +
side, a, b, and c. as in B ; d. inferior tracheal trunk ;
 +
e. transverse branches between upper and lower
 +
tracheal trunks.
 +
 +
 +
 +
410 INSECTA.
 +
 +
 +
 +
gosus (one of the Termites) that peculiar and similar dorsal appendages are
 +
present on the two anterior of the thoracic segments. They are without
 +
tracheae. The anterior atrophies, and the posterior acquires tracheas and gives
 +
rise to the first pair of wings. The second pair of wings is formed from
 +
small processes on the third thoracic segment like those on the other two.
 +
Fritz Miiller concludes from these facts that the wings of Insects are
 +
developed from dorsal processes of the body, not equivalent to the ventral
 +
appendages. What the primitive function of these appendages was is not
 +
clear. Fritz Miiller suggests that they may have been employed as respiratory organs in the passage from an aqueous to a terrestrial existence, when
 +
the Termite ancestors lived in moist habitations a function for which processes supplied with blood-channels would be well adapted. The undoubted
 +
affinity of Insects to Myriapods, coupled with the discovery by Moseley of a
 +
tracheal system in Peripatus, is however nearly fatal to the view that Insects
 +
can have sprung directly from aquatic ancestors not provided with tracheae.
 +
But although this suggestion of Fritz Miiller cannot be accepted, it is still
 +
possible that the processes discovered by him may have been the earliest
 +
rudiments of wings, which were employed first as organs of propulsion by a
 +
water-inhabiting Insect ancestor which had not yet acquired the power of
 +
flying.
 +
 +
The nervous system. The nervous system arises entirely
 +
from the epiblast; but the development of the prae-oral and
 +
post-oral sections may be best considered separately.
 +
 +
The post-oral section, or ventral cord of the adult, arises as
 +
two longitudinal thickenings of the epiblast, one on each side of
 +
the median line (fig. 189 B, vn), which are subsequently split ofif
 +
from the superficial skin and give rise to the two lateral strands
 +
of the ventral cord. At a later period they undergo a differentiation into ganglia and connecting cords.
 +
 +
Between these two embryonic nerve cords there is at first a shallow
 +
furrow, which soon becomes a deep groove (fig. 189 C). At this stage the
 +
differentiation of the lateral elements into ganglia and commissures takes
 +
place, and, according to Hatschek (No. 414), the median groove becomes in
 +
the region of the ganglia converted into a canal, the walls of which soon fuse
 +
with those of the ganglionic enlargements of the lateral cords, and connect
 +
them across the middle line. Between the ganglia on the other hand the
 +
median groove undergoes atrophy, becoming first a solid cord interposed
 +
between the lateral strands of the nervous system, and finally disappearing
 +
without giving rise to any part of the nervous system. It is probable that
 +
Hatschek is entirely mistaken about the entrance of a median element into
 +
the ventral cord, and that the appearances he has described are due to
 +
shrinkage. In Spiders the absence of a median element can be shewn with
 +
great certainty, and, as already stated, this element is not present in
 +
 +
 +
 +
TRACHEATA.
 +
 +
 +
 +
411
 +
 +
 +
 +
Peripatus. Hatschek states that in the mandibular segment the median
 +
element is absorbed, and that the two lateral cords of that part give rise to
 +
the oesophageal commissures, while the sub-cesophageal ganglion is formed
 +
from the fusion of the ganglia of the two maxillary segments.
 +
 +
The prae-oral portion of the nervous system consists entirely
 +
of the supra-cesophageal ganglion. It is formed, according to
 +
Hatschek, of three parts. Firstly and mainly, of a layer sepa
 +
 +
 +
 +
 +
FIG. 189. THREE TRANSVERSE SECTIONS THROUGH THE EMBRYO OF
 +
HYDROPHILUS. (After Kowalevsky.)
 +
 +
A. Transverse section through the larva represented in fig. 187 A.
 +
 +
B. Transverse section through a somewhat older embryo in the region of one of
 +
the stigmata.
 +
 +
C. Transverse section through the larva represented in fig. 187 B.
 +
 +
vn. ventral nerve cord; am. amnion and serous membrane ; me. mesoblast ; me.s.
 +
somatic mesoblast ; hy. hypoblast (?) ; yk. yolk cells (true hypoblast) ; st. stigma of
 +
trachea.
 +
 +
rated from the thickened inner part of the cephalic lobe on each
 +
side ; secondly, of an anterior continuation of the lateral cords ;
 +
and thirdly, of a pit of skin invaginated on each side close to the
 +
 +
 +
 +
412 IN SECT A.
 +
 +
 +
 +
dorsal border of the antennae. This pit is at first provided with
 +
a lumen, which is subsequently obliterated; while the walls of
 +
the pit become converted into true ganglion cells. The two
 +
supra-cesophageal ganglia remain disconnected on the dorsal
 +
side till quite the close of embryonic life.
 +
 +
The tracheae and salivary glands. The tracheae, as was
 +
first shewn by Butschli (No. 405), arise as independent segmentally arranged paired invaginations of the epiblast (fig. 189 B and
 +
C, st). Their openings are always placed on the outer sides of
 +
the appendages of their segments, where such are present.
 +
 +
Although in the adult stigmata are never found in the space
 +
between the prothorax and head 1 , in the embryo and the larva
 +
tracheal invaginations may be developed in all the thoracic (and
 +
possibly in the three jaw-bearing segments) and in all the
 +
abdominal segments except the two posterior.
 +
 +
In the embryo of the Lepidoptera, according to Hatschek (No. 414),
 +
there are 14 pairs of stigmata, belonging to the 14 segments of the body
 +
behind the mouth ; but Tichomiroff states that Hatschek is in error in
 +
making this statement for the foremost post-oral segments. The last two
 +
segments are without stigmata. In the larvae of Lepidoptera as well as those
 +
of many Hymenoptera, Coleoptera and Diptera, stigmata are present on all
 +
the postcephalic segments except the 2nd and 3rd thoracic and the two last
 +
abdominal. In Apis there are eleven pairs of tracheal invaginations according to Kowalevsky (No. 416), but according to Butschli (No. 405) only ten,
 +
the prothorax being without one. In the Bee they appear simultaneously,
 +
and before the appendages.
 +
 +
The blind ends of the tracheal invaginations frequently (e.g.
 +
Apis) unite together into a common longitudinal canal, which
 +
forms a longitudinal tracheal stem. In other cases (eg. Gryllotalpa, Dohrn, No. 408) they remain distinct, and each tracheal
 +
stem has a system of branches of its own.
 +
 +
The development of the tracheae strongly supports the view,
 +
arrived at by Moseley from his investigations on Peripatus, that
 +
they are modifications of cutaneous glands.
 +
 +
The salivary and spinning glands are epiblastic structures,
 +
which in their mode of development are very similar to the
 +
tracheae, and perhaps have a similar origin. The salivary glands
 +
 +
1 In Smynthurus, one of the Collembola, there are, according to Lubbock, only
 +
two stigmata, which are placed on the head.
 +
 +
 +
 +
TRACHEATA. 413
 +
 +
 +
 +
arise as paired epiblastic imaginations, not, as might be
 +
expected, of the Stomodaeum, but of the ventral plate behind
 +
the mouth on the inner side of the mandibles. At first independent, they eventually unite in a common duct, which falls into
 +
the mouth. The spinning glands arise on the inner side of the
 +
second pair of maxillae in Apis and Lepidoptera, and form
 +
elongated glands extending through nearly the whole length
 +
of the body. They are very similar in their structure and development to salivary glands, and are only employed during larval
 +
life. They no doubt resemble the mucous glands of the oral
 +
papillae of Peripatus, with which they have been compared by
 +
Moseley. The mucous glands of Peripatus may perhaps be the
 +
homologous organs of the first pair of maxillae, for the existence
 +
of which there appears to be some evidence amongst Insects.
 +
 +
Mesoblast. It has been stated that the mesoblast becomes
 +
divided in the region of the body into two lateral bands (fig. 189
 +
A). These bands in many, if not all forms, become divided
 +
into a series of somites corresponding with the segments of the
 +
body. In each of them a cavity appears the commencing
 +
perivisceral cavity which divides them into a somatic plate in
 +
contact with the epiblast, and a splanchnic plate in contact with
 +
the hypoblast (fig. 189). In the interspaces between the
 +
segments the mesoblast is continuous across the median ventral
 +
line. The mesoblast is prolonged into each of the appendages
 +
as these are formed, and in the appendages there is present a
 +
central cavity. By Metschnikoff these cavities are stated to be
 +
continuous, as in Myriapods and Arachnida, with those of the
 +
somites ; but by Hatschek (No. 414) they are stated to be
 +
independent of those in the somites and to be open to the yolk.
 +
 +
The further details of the history of the mesoblast are very imperfectly
 +
known, and the fullest account' we have is that by Dohrn (No. 408) for
 +
Gryllotalpa. It would appear that the mesoblast grows round and encloses
 +
the dorsal side of the yolk earlier than the epiblast. In Gryllotalpa it forms
 +
a pulsating membrane. As the epiblast extends dorsalwards the median
 +
dorsal part of the membrane is constricted off as a tube which forms the
 +
heart. At the same time the free space between the pulsating membrane
 +
and the yolk is obliterated, but transverse passages are left at the lines
 +
between the somites, through which the blood passes from the ventral part of
 +
the body to corresponding openings in the wall of the heart. The greater
 +
part of the membrane gives rise to the muscles of the trunk.
 +
 +
 +
 +
414 INSECTA.
 +
 +
 +
 +
Ventrally the mesoblastic bands soon meet across the median line. The
 +
cavities in the appendages become obliterated and their mesoblastic walls
 +
form the muscles, etc. The cavities in the separate mesoblastic somites also
 +
cease to be distinctly circumscribed.
 +
 +
The splanchnic mesoblast follows the hypoblast in its growth, and gives
 +
rise to the connective tissue and muscular parts of the walls of the alimentary tract. The mesoblastic wall of the proctodaeum is probably formed
 +
independently of the mesoblastic somites. In the head the mesoblast is
 +
stated to form at first a median ventral mass, which does not pass into the
 +
procephalic lobe ; though it assists in forming both the antennae and upper
 +
lip.
 +
 +
The alimentary canal. The alimentary tract of Insects is
 +
formed of three distinct sections (fig. 181) a mesenteron or
 +
middle section (me), a stomodaeum (st) and a proctodaeum (an).
 +
The stomodaeum and proctodaeum are invaginations of the
 +
epiblast, while the mesenteron is lined by the hypoblast. The
 +
distinction between the three is usually well marked in the adult
 +
by the epiblastic derivatives being lined by chitin. The stomodaeum consists of mouth, oesophagus, crop, and proventriculus or
 +
gizzard, when such are present. The mesenteron includes the
 +
stomach, and is sometimes (Orthoptera, etc.) provided at its
 +
front end with pyloric diverticula posteriorly it terminates just
 +
in front of the Malpighian bodies. These latter fall into the
 +
proctodaeum, which includes the whole of the region from their
 +
insertion to the anus.
 +
 +
The oral invagination appears nearly coincidently. with the
 +
first formation of segments at the front end of the groove
 +
between the lateral nerve cords, and the anal invagination
 +
appears slightly later at the hindermost end of the ventral plate.
 +
 +
The Malpighian bodies arise as two pairs of outgrowths of the
 +
epiblast of t/te proctodceum, whether solid at first is not certain.
 +
The subsequent increase which usually takes place in their
 +
number is due to sproutings (at first solid) of the two original
 +
vessels.
 +
 +
The glandular walls of the mesenteron are formed from the hypoblast ;
 +
but the exact origin of the layer has not been thoroughly worked out in all
 +
cases. In Hydrophilus it is stated by Kowalevsky (No. 416) to appear as
 +
two sheets split off from the lateral masses of mesoblast, which gradually
 +
grow round the yolk, and a similar mode of formation would seem to hold
 +
good for Apis. Tichomiroff (No. 420) confirms Kowalevsky on this point,
 +
 +
 +
 +
TR ACHE AT A. 415
 +
 +
 +
 +
and further states that these two masses meet first ventrally and much later
 +
on the dorsal side. In Lepidoptera, on the other hand, Hatschek finds that
 +
the hypoblast arises as a median mass of polygonal cells in the anterior part
 +
of the ventral plate. These cells increase by absorbing material from the
 +
yolk, and then gradually extend themselves and grow round the yolk.
 +
 +
Dohrn (No. 408) believes that the yolk cells, the origin of which has
 +
already been spoken of, give rise to the hypoblastic walls of the mesenteron,
 +
and this view appears to be shared by Graber (No. 412), though the latter
 +
author holds that some of the yolk cells are derived by budding from the
 +
blastoderm 1 .
 +
 +
From the analogy of Spiders I am inclined to accept Dohrn's and
 +
Graber's view. It appears to me probable that Kowalevsky's observations
 +
are to be explained by supposing that the hypoblast plates which he believes
 +
to be split off from the mesoblast are really separated from the yolk.
 +
 +
.It will be convenient to add here a few details to what has already been
 +
stated as to the origin of the yolk cells. As mentioned above, the central
 +
yolk breaks up at a period, which is not constant in the different forms, into
 +
polygonal or rounded masses, in each of which a nucleus has in many
 +
instances been clearly demonstrated although in others such nuclei have not
 +
been made out. It is probable however that nuclei are in all cases really
 +
present, and that these masses must be therefore regarded as cells. They
 +
constitute in fact the yolk cells. The periphery of the yolk breaks up into
 +
cells while the centre is still quite homogeneous.
 +
 +
The hypoblastic walls of the mesenteron appear to be formed
 +
in the first instance laterally (fig. 189 B and C, hy). They then
 +
meet ventrally (fig. 185 A and B), and finally close in the
 +
mesenteron on the dorsal side.
 +
 +
The mesenteron is at first a closed sack, independent of both
 +
stomodaeum and proctodaeum ; and in the case of the Bee it so
 +
remains even after the close of embryonic life. The only glandular organs of the mesenteron are the not unfrequent pyloric
 +
tubes, which are simple outgrowths of its anterior end. It is
 +
possible that in some instances they may be formed in situ
 +
around the lateral parts of the yolk.
 +
 +
In many instances the whole of the yolk is enclosed in the walls of the
 +
mesenteron, but in other cases, as in Chironomus and Simulia (Weismann,
 +
No. 430 ; Metschnikoff, No. 423), part of the yolk may be left between the
 +
ventral wall of the mesenteron and the ventral plate. In Chironomus the
 +
 +
1 Graber's view on this point may probably be explained by supposing that he has
 +
mistaken a passage of yolk cells into the blastoderm for a passage of blastoderm cells
 +
into the yolk. The former occurrence takes place, as I have found, largely in Spiders,
 +
and probably therefore also occurs in Insects.
 +
 +
 +
 +
41 6 INSECTA.
 +
 +
 +
 +
mass of yolk external to the mesenteron takes the form of a median and two
 +
lateral streaks. Some of the yolk cells either prior to the establishment of
 +
the mesenteron, or derived from the unenclosed portions of the yolk, pass
 +
into the developing organs (Dohrn, 408) and serve as a kind of nutritive cell.
 +
They also form blood corpuscles and connective-tissue elements. Such yolk
 +
cells may be compared to the peculiar bodies described by Reichenbach in
 +
Astacus, which form the secondary mesoblast. Similar cells play a very
 +
important part in the development of Spiders.
 +
 +
Generative organs. The observations on the development of the
 +
generative organs are somewhat scanty. In Diptera certain cells known
 +
as the pole cells are stated by both Metschnikoff (No. 423) and Leuckart to
 +
give rise to the generative organs. The cells in question (in Chironomus
 +
and Musca vomitoria, Weismann, No. 430) appear at the hinder end of the
 +
ovum before any other cells of the blastoderm. They soon separate from
 +
the blastoderm and increase by division. In the embryo, produced by the
 +
viviparous larva of Cecidomyia, there is at first a single pole cell, which
 +
eventually divides into four, and the resulting cells become enclosed within
 +
the blastoderm. They next divide into two masses, which are stated by
 +
Metschnikoff (No. 423) to become surrounded by indifferent embryonic cells 1 .
 +
Their protoplasm then fuses, and their nuclei divide, and they give rise to
 +
the larval ovaries, for which the enclosing cells form the tunics.
 +
 +
In Aphis Metschnikoff (No. 423) detected at a very early stage a mass
 +
of cells which give rise to the generative organs. These cells are situated
 +
at the hind end of the ventral plate ; and, except in the case of one of the
 +
cells which gives rise by division to a green mass adjoining the fat body,
 +
the protoplasm of the separate cells fuses into a syncytium. Towards the
 +
close of embryonic life the syncytium assumes a horse-shoe form. The mass
 +
is next divided into two, and the peripheral layer of each part gives rise
 +
to the tunic, while from the hinder extremity of each part an at first solid
 +
duct the egg- tube grows out. The masses themselves form the germogens. The oviduct is formed by a coalescence of the ducts from each
 +
germogen.
 +
 +
Ganin derives the generative organs in Platygaster (vide p. 347) from
 +
the hind end of the ventral plate close to the proctodaeum ; while Suckow
 +
states that the generative organs are outgrowths of the proctodicum.
 +
According to these two sets of observations the generative organs would
 +
appear to have an epiblastic origin an origin which is not incompatible
 +
with that from the pole cells.
 +
 +
In Lepidoptera the genital organs are present in the later periods of
 +
embryonic life as distinct paired organs, one on each side of the heart, in
 +
the eighth postcephalic segment. They are elliptical bodies with a duct
 +
passing off from the posterior end in the female or from the middle in the
 +
male. The egg-tubes or seminal tubes are outgrowths of the elliptical
 +
bodies.
 +
 +
1 This point requires further observation.
 +
 +
 +
 +
TRACHEATA.
 +
 +
 +
 +
417
 +
 +
 +
 +
In other Insects the later stages in the development of the generative
 +
organs closely resemble those in the Lepidoptera, and the organs are usually
 +
distinctly visible in the later stages of embryonic life.
 +
 +
It may probably be laid down, in spite of some of Metschnikoff's
 +
observations above quoted, that the original generative mass gives rise to
 +
both the true genital glands and their ducts. It appears also to be fairly
 +
clear that the genital glands of both sexes have an identical origin.
 +
 +
Special types of larva.
 +
 +
Certain of the Hymenopterous forms, which deposit their eggs in the
 +
eggs or larvae of other Insects, present very peculiar modifications in their
 +
development. Platygaster, which lays its egg in the larvae of Cecidomyia,
 +
undergoes perhaps the most remarkable development amongst these forms.
 +
It has been studied especially by Ganin (No. 410), from whom the following
 +
account is taken.
 +
 +
The very first stages are unfortunately but imperfectly known, and the
 +
interpretations offered by Ganin do not in all cases appear quite satisfactory. In the earliest stage after being laid the egg is enclosed in a
 +
capsule produced into a stalk (fig. 190 A). In the interior of the egg
 +
there soon appears a single spherical body, regarded by Ganin as a cell
 +
(fig. 190 B). In the next stage three similar bodies appear in the vitellus,
 +
no doubt derived from the first one (fig. 190 C). The central one presents
 +
somewhat different characters to the two others, and, according to Ganin,
 +
gives rise to the whole embryo. The two peripheral bodies increase by
 +
division, and soon appear as nuclei imbedded in a layer of protoplasm (fig. 190 D,
 +
E, F). The layer so
 +
formed serves as a
 +
covering for the embryo, regarded by
 +
Ganin as equivalent
 +
to the amnion (? serous membrane) of
 +
other Insect embryos. In the embryo cell new cells
 +
are stated to be
 +
formed by a process
 +
of endogenous cell formation (fig. 190 D, E). It appears probable that
 +
Ganin has mistaken nuclei for cells in the earlier stages, and that a blastoderm is formed as in other Insects, and that this becomes divided in a way
 +
not explained into a superficial layer which gives rise to the serous
 +
envelope, and a deeper layer which forms the embryo. However this
 +
 +
B. II. 27
 +
 +
 +
 +
 +
FlG. 190. A SERIES OF STAGES IN THE DEVELOPMENT
 +
 +
OF PLATYGASTER. (From Lubbock ; after Ganin.)
 +
 +
 +
 +
41 8 INSECTA.
 +
 +
 +
 +
may be, a differentiation into an epiblastic layer of columnar cells and
 +
a hypoblastic layer of more rounded cells soon becomes apparent in the body
 +
of the embryo. Subsequently to this the embryo grows rapidly, till by a
 +
deep transverse constriction on the ventral surface it becomes divided into an
 +
anterior cephalothoracic portion and a posterior caudal portion (fig. 190 F).
 +
The cephalothorax grows in breadth, and near its anterior end an invagination appears, which gives rise to the mouth and cesophagus. On
 +
the ventral side of the cephalothorax there is first formed a pair of
 +
claw-like appendages on each side of the mouth, then a posterior pair of
 +
appendages near the junction of the cephalothorax and abdomen, and
 +
lastly a pair of short conical antennae in front.
 +
 +
At the same time the hind end of the abdomen becomes bifid, and gives
 +
rise to a fork-like caudal appendage ; and at a slightly later period four
 +
grooves make their appearance in the caudal region, and divide this part of
 +
the embryo into successive segments. While these changes have been
 +
taking place in the general form of the embryo, the epiblast has given rise
 +
to a cuticle, and the hypoblastic cells have become differentiated into a
 +
central hypoblastic axis the mesenteron and a surrounding layer of
 +
mesoblast, some of the cells of which form longitudinal muscles.
 +
 +
With this stage closes what may be regarded as the embryonic development of Platygaster. The embryo becomes free from the amnion, and presents itself as a larva, which from its very remarkable characters has been
 +
spoken of as the Cyclops larva by Ganin.
 +
 +
The larvae of three species have been described by Ganin, which are represented in fig. 1 9 1 A, B, C. These larvae are strangely dissimilar to the ordinary
 +
Hexapod type, whether larval or adult. They are formed of a cephalothoracic
 +
shield with the three pairs of appendages (a, kf, lfg\ the development of
 +
which has already been described, and of an abdomen formed of five segments, the last of which bears the somewhat varying caudal appendages.
 +
The nervous system is as yet undeveloped.
 +
 +
The larvae move about in the tissues of their hosts by means of their
 +
claws.
 +
 +
The first larval condition is succeeded by a second with very different
 +
characters, and the passage from the first to the second is accompanied by
 +
an ecdysis.
 +
 +
The ecdysis commences at the caudal extremity, and the whole of the
 +
last segment is completely thrown off. As the ecdysis extends forwards
 +
the tail loses its segmentation and becomes strongly compressed, the
 +
appendages of the cephalothorax are thrown off, and the whole embryo
 +
assumes an oval form without any sharp distinction into different regions
 +
and without the slightest indication of segmentation (fig. 191 D). Of the
 +
internal changes which take place during the shedding of the cuticle, the
 +
first is the formation of a proctodaeum (gfi) by an invagination, which ends
 +
blindly in contact with the mesenteron. Shortly after this a thickening of
 +
the epiblast (bsm} appears along the ventral surface, which gives rise mainly
 +
to the ventral nerve cord ; this thickening is continuous behind with the
 +
 +
 +
 +
TRACHEATA.
 +
 +
 +
 +
419
 +
 +
 +
 +
epiblast which is invaginated to form the proctodaeum, and in front is prolonged on each side into two procephalic lobes, in which there are also
 +
thickenings of the epiblast (gsae), which become converted into supraoesophageal ganglia, and possibly other parts.
 +
 +
Towards the close of the second larval period the muscles (/;) become
 +
segmentally arranged, and give indications of the segmentation which
 +
 +
 +
 +
 +
FlG. 191. A SERIES OF STAGES IN THE DEVELOPMENT OF PLATYGASTER.
 +
 +
(From Lubbock ; after Ganin.)
 +
 +
A. B. C. Cyclops larvae of three species of Platygaster.
 +
D. Second larval stage. E. Third larval stage.
 +
 +
mo. mouth ; a. antenna ; kf. hooked feet ; Ifg. lateral feet ; /. branches of tail ;
 +
ul. lower lip ; slkf. oesophagus ; gsae. supra- oesophageal ganglion ; bsm. ventral epiblastic plate ; Im. lateral muscles (the letters also point in D to the salivary glands) ;
 +
gh. proctodseum ; ga. generative organs ; md. mandibles ; ag. ducts of salivary glands ;
 +
sp. (in E) salivary glands ; mis. stomach ; ed. intestine ; ew. rectum ; ao. anus ;
 +
tr. tracheae ; fk. fat body.
 +
 +
becomes apparent in the third larval period. The third and last larval
 +
stage (fig. 191 E) of Platygaster, during which it still remains in the tissues
 +
of its host, presents no very peculiar features. The passage from the second
 +
to the third form is accompanied by an ecdysis.
 +
 +
Remarkable as are the larvae just described, there can I think be
 +
no reason, considering their parasitic habits, for regarding them as ancestral.
 +
 +
272
 +
 +
 +
 +
420 INSECTA.
 +
 +
 +
 +
Metamorphosis and heterogamy.
 +
 +
Metamorphosis. The majority of Insects are born in a
 +
condition in which they obviously differ from their parents. The
 +
extent of this difference is subject to very great variations, but
 +
as a rule the larvae pass through a very marked metamorphosis
 +
before reaching the adult state. The complete history of this
 +
metamorphosis in the different orders of Insects involves a far
 +
too considerable amount of zoological detail to be dealt with in
 +
this work ; and I shall confine myself to a few observations on
 +
the general characters and origin of the metamorphosis, and of
 +
the histological processes which take place during its occurrence 1 .
 +
 +
In the Aptera the larva differs from the adult only in the
 +
number of facets in the cornea and joints in the antennae.
 +
 +
In most Orthoptera and Hemiptera the larvae differ from the
 +
adult in the absence of wings and in other points. The wings,
 +
etc., are gradually acquired in the course of a series of successive
 +
moultings. In the Ephemeridae and Libellulidae, however, the
 +
metamorphosis is more complicated, in that the larvae have
 +
provisional tracheal gills which are exuviated before the final
 +
moult. In the Ephemeridae there are usually a great number of
 +
moultings ; the tracheal gills appear after the second moult, and
 +
the rudiments of the wings when the larva is about half grown.
 +
Larval life may last for a very long period.
 +
 +
In all the other groups of Insects, viz. the Diptera, Neuroptera, Coleoptera, Lepidoptera, and Hymenoptera, the larva
 +
passes with a few exceptions through a quiescent stage, in
 +
which it is known as a pupa, before it attains the adult stage.
 +
These forms are known as the Holometabola.
 +
 +
In the Diptera the larvae are apodous. In the true flies (Muscidae) they
 +
are without a distinct head and have the jaws replaced by hooks. In the
 +
Tipulidae there is on the other hand a well-developed head with the normal
 +
appendages. The pupae of the Muscidae are quiescent, and are enclosed in
 +
the skin of the larva which shrinks and forms a firm oval case. In the
 +
 +
1 For a systematic account of this subject the reader is referred to Lubbock (No.
 +
420) and to Graber (No. 411). He will find in Weismann (Nos. 430 and 431) a detailed
 +
account of the internal changes which take place.
 +
 +
 +
 +
TRACHEATA. 42 1
 +
 +
 +
 +
Tipulidae the larval skin is thrown off at the pupa stage, and in some cases
 +
the pupae continue to move about.
 +
 +
The larvae of the Neuroptera are hexapodous voracious forms. When the
 +
larva becomes a pupa all the external organs of the imago are already
 +
established. The pupa is often invested in a cocoon. It is usually quiescent,
 +
though sometimes it begins to move about shortly before the imago emerges.
 +
 +
In the Coleoptera there is considerable variety in the larval forms. As a
 +
rule the larvae are hexapodous and resemble wingless Insects. But some
 +
herbivorous larvae (e.g. the larva of Melolontha) closely resemble true
 +
caterpillars, and there are also grub-like larvae without feet (Curculio) which
 +
resemble the larvae of Hymenoptera. The pupa is quiescent, but has all
 +
the parts of the future beetle plainly visible. The most interesting larvae
 +
among the Coleoptera are those of Sitaris, one of the Meloidae (Fabre, No.
 +
409). They leave the egg as active hexapodous larvae which attach themselves to the bodies of Hymenoptera, and are thence transported to a cell
 +
filled with honey. Here they eat the ovum of the Hymenopterous form.
 +
They then undergo an ecdysis, in which they functionally lose their appendages, retaining however small rudiments of them, and become grubs. They
 +
feed on the honey and after a further ecdysis become pupae.
 +
 +
In the Lepidoptera the larva has the well-known form of a caterpillar.
 +
The caterpillars have strong jaws, adapted for biting vegetable tissues,
 +
which are quite unlike the oral appendages of the adult. They have three
 +
pairs of jointed thoracic legs, and a variable number (usually five) of pairs
 +
of rudimentary abdominal legs the so-called pro-legs. The larva undergoes
 +
numerous ecdyses, and the external parts of the adult such as the wings, etc.,
 +
are formed underneath the chitinous exoskeleton before the pupa stage.
 +
The pupa is known as a chrysalis and in some Lepidoptera is enveloped in
 +
a cocoon.
 +
 +
The Hymenoptera present considerable variations in the character of the
 +
larvae. In the Aculeata, many Entomophaga, the Cynipidae, etc., the larvae
 +
are apodous grubs, incapable of going in search of their food ; but in the
 +
Siricidse they are hexapodous forms like caterpillars, which are sometimes
 +
even provided with pro-legs. In some of the Entomophaga the larvae have
 +
very remarkable characters which have already been described in a special
 +
section, 'vide pp. 418, 419.
 +
 +
Before proceeding to the consideration of the value of the
 +
various larval forms thus shortly enumerated, it is necessary to
 +
say a few words as to the internal changes which take place
 +
during the occurrence of the above metamorphosis. In the
 +
simplest cases, such as those of the Orthoptera and Hemiptera,
 +
where the metamorphosis is confined to the gradual formation
 +
of the wings, etc. in a series of moults, the wings first appear as
 +
two folds of the epidermis beneath the cuticle on the two
 +
posterior thoracic segments. At the next moult these processes
 +
 +
 +
 +
422 INSECTA.
 +
 +
 +
 +
become covered by the freshly formed cuticle, and appear as
 +
small projections. At every successive moult these projections
 +
become more prominent owing to a growth in the epidermis
 +
which has taken place in the preceding interval. Accompanying
 +
the formation of such organs as the wings, internal changes
 +
necessarily take place in the arrangement of the muscles, etc. of
 +
the thorax, which proceed pari passu with the formation of the
 +
organs to which they belong. The characters of the metamorphosis in such forms as the Ephemeridae only differ from the
 +
above in the fact that provisional organs are thrown off at the
 +
same time that the new ones are formed.
 +
 +
In the case of the Holometabola the internal phenomena of
 +
the metamorphosis are of a very much more remarkable character. The details of our knowledge are largely due to Weismann (Nos. 430 and 431). The larvae of the Holometabola have
 +
for the most part a very different mode of life to the adults.
 +
A simple series of transitions between the two is impossible,
 +
because intermediate forms would be for the most part incapable
 +
of existing. The transition from the larval to the adult state is
 +
therefore necessarily a more or less sudden one, and takes place
 +
during the quiescent pupa condition. Many of the external
 +
adult organs are however formed prior to the pupa stage, but do
 +
not become visible on the surface. The simplest mode of Holometabolic metamorphosis may be illustrated by the development
 +
of Corethra plumicornis, one of the Tipulidae. This larva, like
 +
that of other Tipulidae, is without thoracic appendages, but
 +
before the last larval moult, and therefore shortly before the
 +
pupa stage, certain structures are formed, which Weismann has
 +
called imaginal discs. These imaginal discs are in Corethra
 +
simply invaginations of the epidermis. There are in the thorax
 +
six pairs of such structures, three dorsal and three ventral. The
 +
three ventral are attached to the terminations of the sensory
 +
nerves, and the limbs of the imago are formed as simple
 +
outgrowths of them, which as they grow in length take a spiral
 +
form. In the interior of these outgrowths are formed the
 +
muscles, tracheae, etc., of the limbs; which are believed by
 +
Weismann (it appears to me without sufficient ground) to be
 +
derived from a proliferation of the cells of the neurilemma.
 +
The wings are formed from the two posterior dorsal imaginal
 +
 +
 +
 +
TRACHEATA. 423
 +
 +
 +
 +
discs. The hypodermis of the larva passes directly into that of
 +
the imago.
 +
 +
The pupa stage of Corethra is relatively very short, and the
 +
changes in the internal parts which take place during it are not
 +
considerable. The larval abdominal muscles pass for the most
 +
part unchanged into those of the imago, while the special
 +
thoracic muscles connected with the wings, etc., develop directly
 +
during the latest larval period from cords of cells already formed
 +
in the embryo.
 +
 +
In the Lepidoptera the changes in the passage from the
 +
larval to the adult state are not very much more considerable
 +
than those in Corethra. Similar imaginal discs give rise during
 +
the later larval periods to the wings, etc. The internal changes
 +
during the longer pupa period are somewhat more considerable.
 +
Important modifications and new formations arise in connection with the alimentary tract, the nervous and muscular
 +
systems.
 +
 +
The changes which take place in the true flies (Muscidse) are
 +
far more complicated than either those in Corethra or in the
 +
Lepidoptera. The abdomen of the larva of Musca becomes
 +
bodily converted into the abdomen of the imago as in the above
 +
types, but the whole epidermis and appendages of the head and
 +
thorax are derived from imaginal discs which are formed within
 +
and (so far as is known) independently of the epidermis of the
 +
larva or embryo. These imaginal discs are simple masses of
 +
apparently indifferent cells, which for the most part appear at
 +
the close of embryonic life, and are attached to nerves or
 +
tracheae. They grow in size during larval life, but during the
 +
relatively long pupa stage they unite together to give rise to a
 +
continuous epidermis, from which the appendages grow out as
 +
processes. The epidermis of the anterior part of the larva is
 +
simply thrown off, and has no share in forming the epidermis of
 +
the adult.
 +
 +
There are a pair of cephalic imaginal discs and six pairs of
 +
thoracic discs. Two pairs, a dorsal and a ventral, give rise to
 +
each thoracic ring, and the appendages attached to it.
 +
 +
Though, as mentioned above, no evidence has yet been
 +
produced to shew that the imaginal discs of Musca are derived
 +
from the embryonic epiblast, yet their mode of growth and
 +
 +
 +
 +
424 1NSECTA.
 +
 +
 +
 +
eventual fate proves beyond the shadow of a doubt that they are
 +
homologous with the imaginal discs of Corethra. Their earliest
 +
origin is well worth further investigation.
 +
 +
The metamorphosis of the internal organs is still more
 +
striking than that of the external. There is a disruption, total
 +
or partial, of all the internal organs except the generative
 +
organs. In the case of the alimentary tract, the Malpighian
 +
vessels, the heart and the central nervous system, the disruption
 +
is of a partial kind, which has been called by Weismann
 +
histolysis. The cells of these organs undergo a fatty degeneration, the nuclei alone in some cases remaining. The kind of
 +
plasma resulting from this degeneration retains the shape of the
 +
organs, and finally becomes built up again into the corresponding organs of the imago. The tracheae, muscles and peripheral
 +
nerves, and an anterior part of the alimentary tract, are entirely
 +
disrupted. They seem to be formed again from granular cells
 +
derived from the enormous fat body.
 +
 +
The phenomena of the development of the Muscidse are undoubtedly of
 +
rather a surprising character. Leaving for the moment the question of the
 +
origin of the pupa stage to which I return below, it will be admitted on all
 +
hands that during the pupa stage the larva undergoes a series of changes
 +
which, had they taken place by slow degrees, would have involved, in such a
 +
case as Musca, a complete though gradual renewal of the tissues. Such
 +
being the case, the cells of the organs common to the larva and the imago
 +
would, in the natural course of things, not be the same cells as those of the
 +
larva but descendants of them. We might therefore expect to find in the
 +
rapid conversion of the larval organs into those of the adult some condensation, so to speak, of the process of ordinary cell division. Such condensations
 +
are probably represented in the histolysis in the case of the internal organs,
 +
and in the formation of imaginal discs in the case of the external ones, and
 +
I think it probable that further investigation will shew that the imaginal
 +
discs of the Muscidae are derivatives of the embryonic epiblast. The above
 +
considerations by no means explain the whole of Weismann's interesting
 +
observations, but an explanation is I believe to be found by following up
 +
these lines.
 +
 +
More or less parallel phenomena to those in Insects are found in the
 +
development of the Platyelminthes and Echinoderms. The four disc-like
 +
invaginations of the skin in many larval Nemertines (vide p. 198), which
 +
give rise to the permanent body wall of the Nemertine, may be compared to
 +
the imaginal discs. The subsequent throwing off of the skin of Pilidium or
 +
larva of Desor is a phenomenon like the absorption of part of the larval
 +
skin of Musca. The formation of an independent skin within the first larval
 +
 +
 +
 +
TR ACHE AT A.
 +
 +
 +
 +
425
 +
 +
 +
 +
form in the Distomeaeand in the Cestoda may be compared to the apparently
 +
independent formation of the imaginal discs in Musca.
 +
 +
The fact that in a majority of instances it is possible to trace
 +
an intimate connection between the surroundings of a larva and
 +
its organization proves in the clearest way that the characters of
 +
the majority of existing larval forms of Insects have owed their
 +
origin to secondary adaptations. A few instances will illustrate
 +
this point.
 +
 +
In the simplest types of metamorphosis, e.g. those of the
 +
Orthoptera genuina, the larva has precisely the same habits as
 +
the adult. We find that a caterpillar
 +
form is assumed by phytophagous larvae
 +
amongst the Lepidoptera, Hymenoptera
 +
and Coleoptera. Where the larva has
 +
not to go in search of its nutriment the
 +
grub-like apodous form is assumed. The
 +
existence of such an apodous larva is
 +
especially striking in the Hymenoptera,
 +
in that rudiments of thoracic and abdominal appendages are present in the
 +
embryo and disappear again in the larva.
 +
The case of the larva of Sitaris, already
 +
described (p. 421), affords another very
 +
striking proof that the organization of
 +
the larva is adapted to its habits.
 +
 +
It follows from the above that the
 +
development of such forms as the Orthoptera genuina is more primitive than
 +
that of the holometabolous forms; a
 +
conclusion which tallies with the fact
 +
 +
 +
 +
 +
FIG. 102.
 +
 +
 +
 +
ANTERIOR
 +
 +
 +
 +
HALF OF CAMPODEA FRAGILIS. (From Gegenbaur; after Palmen.)
 +
 +
a. antennae ; p. feet ; j> ',
 +
post-tho
 +
feet; s.
 +
 +
 +
 +
stigma.
 +
 +
that both palaeontological and anatomical evidence shew the
 +
Orthoptera to be a very primitive group of Insects.
 +
 +
The above argument probably applies with still greater force
 +
to the case of the Thysanura ; and it seems to be probable that
 +
this group is more nearly related than any other to the primitive
 +
wingless ancestors of Insects 1 . The characters of the oral
 +
 +
1 Brauer and Lubbock (No. 421) have pointed out the primitive characters of these
 +
forms, especially of Campodea.
 +
 +
 +
 +
426 INSECTA.
 +
 +
 +
 +
appendages in this group, the simplicity of their metamorphosis,
 +
and the presence of abdominal appendages (fig. 192), all tell in
 +
favour of this view, while the resemblance of the adult to the
 +
larvae of the Pseudoneuroptera, etc., points in the same direction.
 +
The Thysanura and Collembola are not however to be regarded
 +
as belonging to the true stock of the ancestors of Insects, but as
 +
degenerated relations of this stock ; much as Amphioxus and
 +
the Ascidians are degenerate relations of the ancestral stock of
 +
Vertebrates, and Peripatus of that of the Tracheata. It is
 +
probable that all these forms have succeeded in retaining their
 +
primitive characters from their degenerate habits, which prevented them from entering into competition in the struggle for
 +
existence with their more highly endowed relatives. While in a
 +
general way it is clear that the larval forms of Insects cannot be
 +
expected to throw much light on the nature of Insect ancestors,
 +
it does nevertheless appear to me probable that such forms as
 +
the caterpillars of the Lepidoptera are not without a meaning in
 +
this respect. It is easy to conceive that even a secondary larval
 +
form may have been produced by the prolongation of one of the
 +
embryonic stages ; and the general similarity of a caterpillar to
 +
Peripatus, and the retention by it of post-thoracic appendages, are
 +
facts which appear to favour this view of the origin of the caterpillar form.
 +
 +
The two most obscure points which still remain to be dealt
 +
with in the metamorphosis of Insects are (i) the origin of the
 +
quiescent pupa stage ; (2) the frequent dissimilarity between the
 +
masticatory apparatus of the larva and adult.
 +