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

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(358) R. v. Willemoes-Suhm. " Biologische Beobachtungen U. niedrige  
 
(358) R. v. Willemoes-Suhm. " Biologische Beobachtungen U. niedrige  
Meeresthiere." Zeit. f. wiss. Zool. Bd. xxi. 1871.  
+
Meeresthiere." Zeit. f. wiss. Zool. Bd. xxi. 1871.
 
 
 
 
==Chapter XIII. Discophora==
 
 
 
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.
 
 
 
These two points may be conveniently dealt with together,
 
and some valuable remarks about them will be found in Lubbock
 
(No. 420).
 
 
 
On grounds already indicated it may be considered certain
 
that the groups of Insects without a pupa stage, and with a larva
 
very similarly organised to the adult, preceded the existing
 
holometabolic groups. The starting-point in the metamorphosis
 
of the latter groups was therefore something like that of the
 
Orthoptera. Suppose it became an advantage to a species that
 
the larva and adult should feed in a somewhat different way, a
 
difference in the character of their mouth parts would soon make
 
itself manifest ; and, since an intermediate type of mouth parts
 
 
 
 
 
 
 
TRACHEATA. 427
 
 
 
 
 
 
 
would probably be disadvantageous, there would be a tendency
 
to concentrate into a single moult the transition from the larval
 
to the adult form of mouth parts. At each ordinary moult there
 
is a short period of quiescence, and this period of quiescence
 
would naturally become longer in the important moult at which
 
the change in the mouth parts was effected. In this way a
 
rudimentary pupa stage might be started. The pupa stage,
 
once started, might easily become a more important factor in
 
the metamorphosis. If the larva and imago diverged still more
 
from each other, a continually increasing amount of change
 
would have to be effected at the pupa stage. It would probably
 
be advantageous to the species that the larva should not have
 
rudimentary functionless wings ; and the establishment of the
 
wings as external organs would therefore become deferred to
 
the pupa stage. The same would probably apply to other
 
organs.
 
 
 
Insects usually pass through the pupa stage in winter in cold
 
climates and during the dry season in the tropics, this stage
 
serving therefore apparently for the protection of the species
 
during the inclement season of the year. These facts are easily
 
explained on the supposition that the pupa stage has become
 
secondarily adapted to play a part in the economy of the
 
species quite different from that to which it owes its origin.
 
 
 
Heterogamy. The cases of alternations of generations
 
amongst Insects all fall under the heading already defined in
 
the introduction as Heterogamy. Heterogamy amongst Insects
 
has been rendered possible by the existence of parthenogenesis,
 
which, as stated in the introduction, has been taken hold of by
 
natural selection, and has led to the production of generations of
 
parthenogenetic forms, by which a clear economy in reproduction
 
is effected. Parthenogenesis without heterogamy occurs in a
 
large number of forms. In Bees, Wasps, and a Sawfly (Nematus
 
ventricosus) the unfertilized ova give rise to males. In two
 
Lepidopterous genera (Psyche and Solenobia) the unfertilized
 
ova give rise mainly, if not entirely, to females. Heterogamy
 
occurs in none of the above types, but in Psyche and Solenobia
 
males are only occasionally found, so that a series of generations
 
producing female young from unfertilized ova are followed by a
 
generation producing young of both sexes from fertilized ova. It
 
 
 
 
 
 
 
428 INSECTA.
 
 
 
 
 
 
 
would be interesting to know if the unimpregnated female would
 
not after a certain number of generations give rise to both males
 
and females ; such an occurrence might be anticipated on
 
grounds of analogy. In the cases of true heterogamy parthenogenesis has become confined to special generations, which differ
 
in their character from the generations which reproduce themselves sexually. The parthenogenetic generations generally
 
flourish during the season when food is abundant; while the
 
sexual generations occur at intervals which are often secondarily
 
regulated by the season, supply of food, etc.
 
 
 
A very simple case of this kind occurs, if we may trust the
 
recent researches of Lichtenstein 1 , in certain Gall Insects
 
(Cynipidae). He finds that the female of a form known as
 
Spathegaster baccarum, of which both males and females are
 
plentiful, pricks a characteristic gall in certain leaves, in which
 
she deposits the fertilized eggs. The eggs from these galls give
 
rise to a winged and apparently adult form, which is not, however, Spathegaster, but is a species considered to belong to a
 
distinct genus known as Neuroterus ventricularis. Only females
 
of Neuroterus are found, and they lay unfertilized ova in peculiar
 
galls which develop into Spathegaster baccarum. Here we have
 
a true case of heterogamy, the females which produce parthenogenetically having become differentiated from those which produce sexually. Another interesting type of heterogamy is that
 
which has been long known in the Aphides. In the autumn
 
impregnated eggs are deposited by females, which give rise in
 
the course of the spring to females which produce parthenogenetically and viviparously. The viviparous females always
 
differ from the females which lay the fertilized eggs. The generative organs are of course differently constituted, and the ova of
 
the viviparous females are much smaller than those of the oviparous females, as is generally the case in closely allied viviparous and oviparous forms; but in addition the former are
 
usually without wings, while the latter are winged. The reverse
 
is however occasionally the case. An indefinite number of generations of viviparous females may be produced if they are artificially kept warm and supplied with food ; but in the course of
 
 
 
1 Petites Nouvelles Entomolog iyues, May, 1878.
 
 
 
 
 
 
 
TRACHEATA. 429
 
 
 
 
 
 
 
nature the viviparous females produce in the autumn males and
 
females which lay eggs with firm shells, and so preserve the
 
species through the winter. The heterogamy of the allied
 
Coccidae is practically the same as that of the Aphidae. In the
 
case of Chermes and Phylloxera the parthenogenetic generations
 
lay their eggs in the normal way.
 
 
 
The complete history of Phylloxera quercus has been worked
 
out by Balbiani (No. 401). The apterous females during the
 
summer lay eggs developing parthenogenetically into apterous
 
females, which continue the same mode of reproduction. In the
 
autumn, however, the eggs which are laid give rise in part to
 
winged forms and in part to apterous forms. Both of these
 
forms lay smaller and larger eggs, which develop respectively
 
into very minute males and females without digestive organs.
 
The fertilized eggs laid by these forms probably give rise to the
 
parthenogenetic females.
 
 
 
A remarkable case of heterogamy accompanied by paedogenesis was discovered by Wagner to take place in certain
 
species of Cecydomyia (Miastor), a genus of the Diptera. The
 
female lays a few eggs in the bark of trees, etc. These eggs
 
develop in the winter into larvae, in which ovaries are early
 
formed. The ova become detached into the body cavity,
 
surrounded by their follicles, and grow at the cost of the
 
follicles. They soon commence to undergo a true development,
 
and on becoming hatched they remain for some time in the
 
body cavity of the parent, and are nourished at the expense of
 
its viscera. They finally leave the empty skin of their parent,
 
and subsequently reproduce a fresh batch of larvae in the same
 
way. After several generations the larvae undergo in the
 
following summer a metamorphosis, and develop into the sexual
 
form.
 
 
 
Another case of paedogenesis is that of the larvae of Chironomus, which have been shewn by Grimm (No. 413) to lay eggs
 
which develop exactly in the same way as fertilized eggs into
 
larvae.
 
 
 
BIBLIOGRAPHY.
 
 
 
(401) M. Balbiani. " Observations s. la reproduction d. Phylloxera du Chene."
 
An. Sc. Nat. Ser. v. Vol. xix. 1874.
 
 
 
 
 
 
 
430 INSECTA.
 
 
 
 
 
 
 
(402) E. Bess els. " Studien u. d. Entwicklung d. Sexualdriisen bei den Lepidoptera." Ztit.f. wiss. Zool. Bd. xvii. 1867.
 
 
 
(403) Alex. Brandt. "Beitrage zur Entwicklungsgeschichte d. Libellulida u.
 
Hemiptera, mil besonderer Berucksichtigung d. Embryonalhiillen derselben." Mem.
 
Ac. Petersbourg, Ser. vn. Vol. xm. 1869.
 
 
 
(404) Alex. Brandt. Ueber das Ei u. seine Bildungsstdttt. Leipzig, 1878.
 
 
 
(405) O. Biitschli. "Zur Entwicklungsgeschichte d. Biene." Zeit. f. wiss.
 
Zool. Bd. xx. 1870.
 
 
 
(406) H. Dewitz. "Bau u. Entwicklung d. Stachels, etc." Zeit.f. wiss. Zool.
 
Vols. xxv. and xxvin. 1875 and 1877.
 
 
 
(407) H. Dewitz. "Beitrage zur Kenntniss d. Postembryonalentwicklung d.
 
Gliedmassen bei den Insecten." Zeit.f. wiss. Zool. xxx. Supplement. 1878.
 
 
 
(408) A. Dohrn. "Notizen zur Kenntniss d. Insectenentwicklung." Zeitschrift
 
f. wiss. Zool. Bd. xxvi. 1876.
 
 
 
(409) M. Fabre. " L'hypermetamorphose et lesmoeursdes Meloides." An.Sci.
 
Nat. Series iv. Vol. vn. 1857.
 
 
 
(410) Ganin. " Beitrage zur Erkenntniss d. Entwicklungsgeschichte d. Insecten."
 
Zeit.f. wiss. Zool. Bd. xix. 1869.
 
 
 
(411) V. Graber. Die Insecten. MUnchen, 1877.
 
 
 
(412) V. Graber. "Vorlauf. Ergeb. lib. vergl. Embryologie d. Insecten."
 
Archivf. mikr. Anat. Vol. XV. 1878.
 
 
 
(413) O. v. Grimm. " Ungeschlechtliche Fortpflanzung einer Chironomus Art-u.
 
deren Entwicklung aus dem unbefruchteten Ei." Mem. Acad. Petersbourg. 1870.
 
 
 
(414) B. Hatschek. " Beitrage zur Entwicklung d. Lepidopteren." Jenaische
 
Zeitschrift, Bd. XI.
 
 
 
(415) A. K 6 1 1 i k e r. " Observationes de prima insectorum genese, etc. " Ann. Sc.
 
Nat. Vol. xx. 1843.
 
 
 
(416) A. Kowalevsky. " Embryologische Studien an Wurmern u. Arthropoden."
 
Mem. Ac. imp. Petersbourg, Ser. vn. Vol. xvi. 1871.
 
 
 
(417) C. Kraepelin. 4 ' Untersuchungen Ub. d. Bau, Mechanismus u. d. Entwick. des Stachels d. bienartigen Thiere." Zeit.f. wiss. Zool. Vol. xxni. 1873.
 
 
 
(418) C. Kupffer. "Faltenblatt an d. Embryonen d. Gattung Chironomus."
 
Arch.f. mikr. Anat. Vol. u. 1866.
 
 
 
(419) R. Leuckart. Zur Kenntniss d. Generationswechsels u. d. Parthenogenese
 
b. d. Insecten. Frankfurt, 1858.
 
 
 
(420) Lubbock. Origin and Metamorphosis of Insects. 1874.
 
 
 
(421) Lubbock. Monograph on Collembola and Thysanura. Ray Society, 1873.
 
 
 
(422) Melnikow. " Beitrage z. Embryonalentwicklung d. Insecten." Archiv
 
f. Naturgeschichte, Bd. xxxv. 1869.
 
 
 
(423) E. Metschnikoff. "Embryologische Studien an Insecten." Zeit. f.
 
wiss. Zool. Bd. xvi. 1866.
 
 
 
(424) P. Meyer. "Ontogenie und Phylogenie d. Insecten." Jenaische Zeitschrift, Vol. x. 1876.
 
 
 
(425) FritzMiiller. " Beitrage z. Kenntniss d. Termiten." Jenaische Zeitschrift, Vol. IX. 1875.
 
 
 
(426) A. S. Packard. " Embryological Studies on Diplex, Perithemis, and
 
the Thysanurous genus Isotoma." Mem. Peabody Acad. Science, I. i. 1871.
 
 
 
(427) Suckow. " Geschlechtsorgane d. Insecten." Ileusinger's Zeitschrift f.
 
organ. Physik, Bd. n. 1828.
 
 
 
 
 
 
 
TRACHEATA.
 
 
 
 
 
 
 
431
 
 
 
 
 
 
 
(428) Tichomiroff. " Ueber die Entwicklungsgeschichte des Seidenwiirms."
 
Zoologischer Anzeiger, n. Jahr. No. 20 (Preliminary Notice).
 
 
 
(429) Aug. Weismann. "Zur Embryologie d. Insecten." Archiv f. Anat.
 
und Phys. 1864.
 
 
 
(430) Aug. Weismann. " Entwicklung d. Dipteren." Zeit. f. wiss. Zool.
 
Vols. xin. and xiv. Leipzig, 1863 4.
 
 
 
(431) Aug. Weismann. " Die Metamorphose d. Corethra plumicornis. " Zeit.
 
f. wiss. Zool. Vol. xvi. 1866.
 
 
 
(432) N. Wagner. "Beitrag z. Lehre d. Fortpflanzung d. Insectenlarven."
 
Zeit.f. wiss. Zool. Vol. xin. 1860.
 
 
 
(433) Zaddach. Untersuchungen iib. d. Bau u. d. Entwicklungd. Gliederthiere.
 
Berlin, 1854.
 
 
 
ARACHNIDA 1 .
 
 
 
The development of several divisions of this interesting
 
group has been worked out ; and it will be convenient to deal in
 
the first instance with the special history of each of these
 
divisions, and then to treat in a
 
separate section the development of the organs for the
 
whole group.
 
 
 
Scorpionidae. The embryonic development always takes
 
place within the female Scorpion. In Buthus it takes place
 
within follicle-like protuberances of the wall of the ovary.
 
In Scorpio also development
 
commences while the egg is
 
still in the follicle, but when the
 
trunk becomes segmented the
 
embryo passes into the ovarian
 
tube. The chief authority for
 
the development of the Scorpionidae is Metschnikoff (No. 434).
 
 
 
At the pole of the ovum facing the ovarian tube there is
 
 
 
 
 
 
 
 
 
 
 
FIG. 193. OVUM OF SCORPION WITH
 
THE ALREADY -FORMED BLASTODERM
 
SHEWING THE PARTIAL SEGMENTATION.
 
(After Metschnikoff.)
 
 
 
bl. blastoderm.
 
 
 
 
 
 
 
1 The classification of the Arachnida adopted in the present work is shewn below.
 
c Scorpionidse. . . ( Tetrapneumones.
 
 
 
Pedipalpi. IL Aranema - JDipneumones.
 
 
 
I. ArthrOgastra. \ Pseudoscorpionidae.
 
 
 
I Soiifugse. in. Acarina,
 
 
 
^ Phalangidse.
 
 
 
 
 
 
 
432
 
 
 
 
 
 
 
SCORPIONID^E.
 
 
 
 
 
 
 
formed a germinal disc which undergoes a partial segmentation
 
(fig. 193 bl). A somewhat saucer-shaped one-layered blastoderm is then formed, which soon becomes thickened in the
 
centre and then divided into two layers. The outer of these
 
is the epiblast, the inner the mesoblast. Beneath the mesoblast
 
there subsequently appear granular cells, which form the
 
commencement of the hypoblast 1 .
 
 
 
During the formation of the blastoderm a cellular envelope is formed
 
round the embryo. Its origin is doubtful, though it is regarded by
 
Metschnikoff as probably derived from the blastoderm and homologous
 
with the amnion of Insects. It becomes double in the later stages (fig. 195).
 
 
 
During the differentiation of the three embryonic layers the
 
germinal disc becomes somewhat pyriform, the pointed end
 
being the posterior. At this extremity there is a special thickening which is perhaps
 
equivalent to the primitive cumulus of Spiders.
 
The germinal disc continues gradually to spread
 
over the yolk, but the
 
original pyriform area is
 
thicker than the remainder, and is marked off
 
anteriorly and posteriorly by a shallow furrow.
 
It constitutes a structure
 
corresponding with the
 
ventral plate of other
 
Tracheata. It soon becomes grooved by a FIG. 194. THREE SURFACE VIEWS OF THE
 
 
 
. A ,. , f VENTRAL PLATE OF A DEVELOPING SCORPION.
 
 
 
shallow longitudinal fur- (After Metschnikoff.)
 
 
 
A. Before segmentation.
 
 
 
B. After five segments have become formed.
 
 
 
C. After the appendages have begun to be
 
 
 
formed.
 
 
 
 
 
 
 
 
 
row (fig. 194 A) which
 
subsequently becomes
 
less distinct. It is then
 
divided by two transverse lines into three parts 2 .
 
 
 
 
 
 
 
1 The origin of the hypoblast cells, if such these cells are, is obscure. Metschnikoff
 
doubtfully derives them from the blastoderm cells ; from my investigations on Spiders
 
it appears to me more probable that they originate in the yolk.
 
 
 
* The exact fate of the three original segments is left somewhat obscure by
 
 
 
 
 
 
 
TRACHEATA.
 
 
 
 
 
 
 
433
 
 
 
 
 
 
 
In succeeding stages the anterior of the three parts is clearly
 
marked out as the procephalic lobe, and soon becomes somewhat
 
broader. Fresh segments are added from before backwards,
 
and the whole ventral plate increases rapidly in length (fig.
 
194 B).
 
 
 
When ten segments have become formed, appendages appear
 
as paired outgrowths of the nine posterior segments (fig. 194 C).
 
The second segment bears the pedipalpi, the four succeeding
 
segments the four ambulatory appendages, and the four hindermost segments smaller provisional appendages which subsequently disappear, with the possible exception of the second.
 
The foremost segment, immediately behind the procephalic
 
lobes, is very small, and still without a rudiment of the chelicerae, which are subsequently formed on it. It would appear
 
from Metschnikoff's figures to
 
be developed later than the
 
other post-oral segments present at this stage. The still
 
unsegmented tail has become
 
very prominent and makes an
 
angle of 180 with the remainder of the body, over the
 
ventral surface of which it is
 
flexed.
 
 
 
By the time that twelve
 
segments are definitely formed, the procephalic region is
 
distinctly bilobed, and in the
 
median groove extending
 
along it the stomodaeum has
 
become formed (fig. 196 A).
 
The chelicerae (ck) appear as
 
small rudiments on the first
 
post-oral segment, and the
 
 
 
 
 
 
 
 
 
FlG. 195. A FAIRLY-ADVANCED EMBRYO OF THE SCORPION ENVELOPED IN
 
 
 
ITS MEMBRANES. (After Metschnikoff. )
 
 
 
ch. chelicerae ; pd. pedipalpi ; p^p 4 .
 
ambulatory appendages ; al>. post-abdomen
 
(tail).
 
 
 
 
 
 
 
Metschnikoff. He believes however that the anterior segment forms the procephalic
 
lobes, the posterior probably the telson and five adjoining caudal segments, and
 
the middle one the remainder of the body. This view does not appear to me quite
 
satisfactory, since on the analogy of Spiders and other Arthropoda the fresh somites
 
ought to be added by a continuous segmentation of the posterior lobe.
 
 
 
B. II. 28
 
 
 
 
 
 
 
 
 
 
 
 
 
434 1 SEUDOSCORPIONID^E.
 
 
 
nerve cords are distinctly differentiated and ganglionated. In
 
the embryonic state there is one ganglion for each segment.
 
The ganglion in the first segment (that bearing the chelicerse) is
 
very small, but is undoubtedly post-oral.
 
 
 
At this stage, by a growth in which all the three germinal
 
layers have a share, the yolk is completely closed in by the
 
blastoderm. It is a remarkable fact with only few parallels, and
 
those amongst the Arthropoda, that the blastopore, or point
 
where the embryonic membranes meet in closing in the yolk, is
 
situated on the dorsal surface of the embryo.
 
 
 
The general relations of the embryo at about this stage are
 
shewn in fig. 195, where the embryo enclosed in its double
 
cellular membrane is seen in a side view. This embryo is about
 
the same age as that seen from the ventral surface in fig. 196 A.
 
 
 
The general nature of the further changes may easily be
 
gathered from an inspection of fig. 196 B and C, but a few
 
points may be noted.
 
 
 
An upper lip or labrum is formed as an unpaired organ in
 
the line between the procephalic lobes. The pedipalpi become
 
chelate before becoming jointed, and the chelicerae also early
 
acquire their characteristic form. Rudimentary appendages
 
appear on the six segments behind the ambulatory legs, five of
 
which are distinctly shewn in fig. 195 ; they persist only on the
 
second segment, where they appear to form the comb-like
 
organs or pectines. The last abdominal segment, Le. that next
 
the tail, is without provisional appendages. The embryonic tail
 
is divided into six segments including the telson (fig. 196 C, ab).
 
The lungs (st) are formed by paired invaginations, the walls of
 
which subsequently become plicated, on the four last segments
 
which bear rudimentary limbs, and simultaneously with the
 
disappearance of the rudimentary limbs.
 
 
 
PseudoscorpionidaB. The development of Qielifer has been investigated by Metschnikoff (436), and although (except that it is provided
 
with tracheae instead of pulmonary sacks) it might be supposed to be closely
 
related to Scorpio, yet in its development is strikingly different.
 
 
 
The eggs after being laid are carried by the female attached to the first
 
segment of the abdomen. The segmentation (vide p. 93) is intermediate
 
between the types of complete and superficial segmentation. The ovum,
 
mainly formed of food-yolk, divides into two, four, and eight equal segments
 
 
 
 
 
 
 
TRACHEATA.
 
 
 
 
 
 
 
435
 
 
 
 
 
 
 
(fig. 197 A). There then appear one or more clear segments on the surface
 
of these, and finally a complete layer of cells is formed round the central
 
yolk spheres (fig. 197 B), which latter subsequently agglomerate into a
 
central mass. The superficial cells form what may be called a blastoderm,
 
which soon becomes divided into two layers (fig. 197 C). There now
 
appears a single pair of appendages (the pedipalpi) (fig. 198 A,/^/), while at
 
the same time the front end of the embryo grows out into a remarkable
 
proboscis-like prominence a temporary upper lip (concealed in the figure
 
 
 
 
 
 
 
 
 
flf
 
 
 
 
 
 
 
ab
 
 
 
 
 
FIG. 196. THREE STAGES IN THE DEVELOPMENT OF THE SCORPION. THE
 
 
 
EMBRYOS ARE REPRESENTED AS IF SEEN EXTENDED ON A PLANE.
 
 
 
(After Metschnikoff.)
 
 
 
ch. chelicerae ; pd. pedipalpi ; p l />*. ambulatory appendages ; pe. pecten ; st.
 
stigmata ; ab. post abdomen (tail).
 
 
 
behind the pedipalpus), and the abdomen (ab) becomes bent forwards towards the ventral surface. In this very rudimentary condition, after undergoing an ecdysis, the larva is hatched, although it still remains attached to
 
its parent. After hatching it grows rapidly, and becomes filled with a
 
peculiar transparent material. The first pair of ambulatory appendages is
 
formed behind the pedipalpi and then the three suceeding pairs, while at the
 
same time the chelicerae appear as small rudiments in front. External signs
 
of segmentation have not yet appeared, but about this period the nervous
 
system is formed. The supra-cesophageal ganglia are especially distinct,
 
and provided with a central cavity, probably formed by an invagination, as
 
in other Arachnida. In the succeeding stages (fig. 198 B) four provisional
 
 
 
282
 
 
 
 
 
 
 
ARANETNA.
 
 
 
 
 
 
 
pairs of appendages (shewn as small knobs at ati] appear behind the ambulatory feet. The abdomen is bent forwards so as to reach almost to the
 
pedipalpi. In the later stages (fig. 198 C) the adult form is gradually
 
attained. The enormous upper lip persists for some time, but subsequently
 
atrophies and is replaced by a normal labrum. The appendages behind the
 
 
 
 
 
 
 
 
 
FIG. igj. SEGMENTATION AND FORMATION OF THE BLASTODERM IN CHELIFER.
 
 
 
(After Metschnikoff.)
 
 
 
In A the ovum is divided into a number of separate segments. In B a number of
 
small cells have appeared (bl) which form a blastoderm enveloping the large yolk
 
spheres. In C the blastoderm has become divided into two layers.
 
 
 
ambulatory feet atrophy, and the tail is gradually bent back into its final
 
position. The segmentation and the gradual growth of the limbs do not call
 
for special description, and the formation of the organs, so far as is known,
 
agrees with other types.
 
 
 
The segmentation of Chthonius is apparently similar to that of Chelifer
 
(Stecker, No. 437).
 
 
 
Phalangidae. Our knowledge of the development of Phalangium is
 
unfortunately confined to the later stages (Balbiani, No. 438). These stages
 
do not appear however to differ very greatly from those of true Spiders.
 
 
 
Araneina. The eggs of true Spiders are either deposited in
 
nests made specially for them, or are carried about by the
 
females. Species belonging to a considerable number of genera,
 
viz. Pholcus, Epeira, Lycosa, Clubione, Tegenaria and Agelcna
 
 
 
 
 
 
 
TRACHEATA.
 
 
 
 
 
 
 
437
 
 
 
 
 
 
 
have been studied by Claparede (No. 442), Balbiani (No. 439),
 
Barrois (No. 441) and myself (No. 440), and the close similarity
 
between their embryos leaves but little doubt that there are no
 
great variations in development within the group.
 
 
 
The ovum is enclosed in a delicate vitelline membrane,
 
enveloped in its turn by a chorion secreted by the walls of the
 
oviduct. The chorion is covered by numerous rounded prominences, and occasionally exhibits a pattern corresponding with
 
the areas of the cells which formed it. The segmentation has
 
already been fully described, pp. 1 18 and 1 19. At its close there
 
is present an enveloping blastoderm formed of a single layer of
 
large flattened cells. The yolk within is formed of a number of
 
 
 
 
 
 
 
' r .v-ii~-cr^ ^H 1
 
 
 
ooo^ o^*afe
 
 
 
 
 
 
 
 
 
Cll
 
 
 
 
 
 
 
ab
 
 
 
 
 
 
 
FIG. 198. THREE STAGES IN THE DEVELOPMENT OF CHELIFKR.
 
 
 
(After Metschnikoff.)
 
pd. pedipalpi ; ab. abdomen ; an.i. anal invagination ; c/i. chelicerse.
 
 
 
large polygonal segments ; each of which is composed of large
 
yolk spherules, and contains a nucleus surrounded by a layer of
 
protoplasm, which is prolonged into stellate processes holding
 
together the yolk spherules. The nucleus, surrounded by the
 
major part of the protoplasm of each yolk cell, appears, as a rule,
 
 
 
 
 
 
 
438 ARANEINA.
 
 
 
 
 
 
 
to be situated not at the centre, but on one side of its yolk
 
segment.
 
 
 
The further description of the development of Spiders applies
 
more especially to Agelena labyrinthica, the species which
 
formed the subject of my own investigations.
 
 
 
The first differentiation of the blastoderm consists in the
 
cells of nearly the whole of one hemisphere becoming somewhat
 
more columnar than those of the other hemisphere, and in the
 
cells of a small area near one end of the thickened hemisphere
 
becoming distinctly more columnar than elsewhere, and two
 
layers thick. This area forms a protuberance on the surface of
 
the ovum, originally discovered by Claparede, and called by him
 
the primitive cumulus. In the next stage the cells of the
 
thickened hemisphere of the blastoderm become still more
 
columnar; and a second area, at first connected by a whitish
 
streak with the cumulus, makes its appearance. In the second
 
area the blastoderm is also more than one cell deep (fig. 199).
 
It will be noticed that the blastoderm, though more than one
 
cell thick over a large part of the ventral surface, is not divided
 
into distinct layers. The second area appears as a white patch
 
and soon becomes more distinct, while the streak continued to
 
it from the cumulus is no longer visible. It is shewn in surface
 
view in fig. 200 A. Though my observations on this stage are
 
not quite satisfactory, yet it appears to me probable that there
 
is a longitudinal thickened ridge of the blastoderm extending
 
from the primitive cumulus to the large white area. The section
 
represented in fig. 199, which I believe to be oblique, passes
 
through this ridge at its most projecting part.
 
 
 
The nuclei of the yolk cells during the above stages multiply
 
rapidly, and cells are formed in the yolk which join the blastoderm ; there can however be no doubt that the main increase in
 
the cells of the blastoderm has been due to the division of the
 
original blastoderm cells.
 
 
 
In the next stage I have been able to observe there is, in the
 
place of the previous thickened half of the blastoderm, a well
 
developed ventral plate with a procephalic lobe in front, a
 
caudal lobe behind, and an intermediate region marked by
 
about three transverse grooves, indicating a division into
 
segments. This plate is throughout two or more rows of
 
 
 
 
 
 
 
TRACHEATA.
 
 
 
 
 
 
 
439
 
 
 
 
 
 
 
 
 
FIG. 199. SECTION THROUGH THE EMBRYO OF AGELENA LABYRINTHICA.
 
 
 
The section is from an embryo of the
 
same age as fig. 200 A, and is represented
 
with the ventral plate upwards. In the
 
ventral plate is seen a keel-like thickening,
 
which gives rise to the main mass of the
 
mesoblast.
 
 
 
yk. yolk divided into large polygonal
 
cells, in several of which nuclei are shewn.
 
 
 
 
 
 
 
cells thick, and the cells
 
which form it are divided into
 
two distinct layers a columnar superficial layer of epiblast
 
cells, and a deeper layer of
 
mesoblast cells (fig. 203 A).
 
In the latter layer there are
 
several very large cells which
 
are in the act of passing from
 
the yolk into the blastoderm.
 
The identification of the structures visible in the previous
 
stage with those visible in
 
the present stage is to a
 
great extent a matter of
 
guess-work, but it appears
 
to me probable that the
 
primitive cumulus is still present as a slight prominence visible
 
in surface views on the caudal lobe, and that the other thickened
 
patch persists as the procephalic lobe. However this may be,
 
the significance of the primitive cumulus appears to be that it is
 
the part of the blastoderm where two rows of cells become first
 
established \
 
 
 
The whole region of the blastoderm other than the ventral
 
plate is formed of a single row of flattened epiblast cells. The
 
yolk retains its original constitution.
 
 
 
By this stage the epiblast and mesoblast are distinctly
 
differentiated, and the homologue of the hypoblast is to be
 
sought for in the yolk-cells. The yolk-cells are not however
 
entirely hypoblastic, since they continue for the greater part of
 
the development to give rise to fresh cells which join the mesoblast.
 
 
 
The Spider's blastoderm now resembles that of an Insect
 
(except for the amnion) after the establishment of the mesoblast,
 
and the mode of origin of the mesoblast in both groups is very
 
similar, in that the longitudinal ridge-like thickening of the
 
 
 
1 Various views have been put forward by Claparfede and Balbiani about the
 
position and significance of the primitive cumulus. For a discussion of which vide
 
self, No. 440.
 
 
 
 
 
 
 
440 AKANEINA.
 
 
 
 
 
 
 
mesoblast shewn in fig. 199 is probably the homologue of the
 
mesoblastic groove of the Insects' blastoderm.
 
 
 
The ventral plate continues to grow rapidly, and at a somewhat later stage (fig. 200 B) there are six segments interposed
 
between the procephalic and caudal lobes. The two anterior of
 
these (ch and pd), especially the foremost, are less distinct than
 
the remainder ; and it is probable that both of them, and in any
 
case the anterior one, are formed later than the three segments
 
following. These two segments are the segments of the chelicenc
 
and pedipalpi. The four segments following belong to the four
 
pairs of ambulatory legs. The segments form raised transverse
 
bands separated by transverse grooves. There is at this stage a
 
faintly marked groove extending along the median line of the
 
ventral plate. This groove is mainly caused by the originally
 
single mesoblastic plate having become divided throughout the
 
whole region of the ventral plate, except possibly the procephalic
 
lobes, into two bands, one on each side of the middle line (fig.
 
203 B).
 
 
 
The segments continue to increase in number by the continuous addition of fresh segments between the one last formed
 
and the caudal lobe. By the stage with nine segments the first
 
rudiments of the limbs make their appearance. The first
 
rudiments to appear are those of the pedipalpi and four ambulatory limbs : the chelicerae, like the segment to which they
 
belong, lag behind in development. The limbs appear as small
 
protuberances at the borders of their segments. By the stage
 
when they are formed the procephalic region has become
 
bilobed, and the two lobes of which it is composed are separated
 
by a shallow groove.
 
 
 
By a continuous elongation the ventral plate comes to form
 
a nearly complete equatorial ring round the ovum, the procephalic and caudal lobes being only separated by a very narrow
 
space, the undeveloped dorsal region of the embryo. This is
 
shewn in longitudinal section in fig. 204. In this condition the
 
embryo may be spoken of as having a dorsal flexure. By the
 
time that this stage is reached (fig. 200 C) the full number of
 
segments and appendages has become established. There are
 
in all sixteen segments (including the caudal lobe). The first
 
six of these bear the permanent appendages of the adult ; the
 
 
 
 
 
 
 
TRACHEATA. 44!
 
 
 
 
 
 
 
next four are provided with provisional appendages ; while the
 
last six are without appendages. The further features of this
 
stage which deserve notice are (i) the appearance of a shallow
 
depression (st) the rudiment of the stomodaeum between the
 
hinder part of the two procephalic lobes ; (2) the appearance of
 
 
 
 
 
 
 
 
 
FIG. aoo. FOUR STAGES IN THE DEVELOPMENT OF AGELENA LABYRINTHICA.
 
 
 
A. Stage when the ventral plate is very imperfectly differentiated, pr.c. primitive
 
cumulus.
 
 
 
B. Ovum viewed from the side when the ventral plate has become divided into
 
six segments, ch. segment of chelicerae imperfectly separated from procephalic lobe ;
 
pd. segment of pedipalpi.
 
 
 
C. Ventral plate ideally unrolled after the full number of segments and
 
appendages are established, st. stomodoeum between the two proe-oral lobes.
 
Behind the six pairs of permanent appendages are seen four pairs of provisional
 
appendages.
 
 
 
D and E. Two views of an embryo at the same stage. D ideally unrolled,
 
E seen from the side. st. stomodseum ; ch. chelicerse ; on their inner side is seen
 
the ganglion belonging to them. pd. pedipalpi ; pr.p. provisional appendages.
 
 
 
raised areas on the inner side of the six anterior appendagebearing segments. These are the rudiments of the ventral
 
ganglia. It deserves to be especially noted that the segment of
 
 
 
 
 
 
 
44 2 AKANEINA.
 
 
 
 
 
 
 
the chelicera, like the succeeding segments, is provided with
 
ganglia ; and that the ganglia of the chelicerae are quite distinct
 
from the supra-cesophageal ganglia derived from the procephalic
 
lobes. (3) The pointed form of the caudal lobe. In Pholcus
 
(Claparede, No. 442) the caudal lobe forms a projecting structure
 
which, like the caudal lobe of the Scorpion, bends forward so as
 
to face the ventral surface of the part of the body immediately
 
in front. In most Spiders such a projecting caudal lobe is not
 
found. While the embryo still retains its dorsal flexure considerable changes are effected in its general constitution. The
 
appendages (fig. 200 D and E) become imperfectly jointed, and
 
grow inwards so as to approach each other in the middle line.
 
Even in the stage before this, the ventral integument between
 
the rudiments of the ganglia had become very much thinner,
 
and had in this way divided the ventral plate into two halves.
 
At the present stage the two halves of the ventral plate are still
 
further separated, and there is a wide space on the ventral side
 
only covered by a delicate layer of epiblast. This is shewn in
 
surface view (fig. 200 D) and in section in fig. 203 C.
 
 
 
The stomodaeum (j/) is much more conspicuous, and is
 
bounded in front by a prominent upper lip, and by a less
 
marked lip behind. The upper lip becomes less conspicuous in
 
later stages, and is perhaps to be compared with the provisional
 
upper lip of Chelifer. Each procephalic lobe is now marked by
 
a deep semicircular groove.
 
 
 
The next period in the development is characterised by the
 
gradual change in the flexure of the embryo from a dorsal to a
 
ventral one ; accompanied by the division of the body into an
 
abdomen and cephalo-thorax, and the gradual assumption of the
 
adult characters.
 
 
 
The change in the flexure of the embryo is caused by the
 
elongation of the dorsal region, which has hitherto been hardly
 
developed. Such an elongation increases the space on the
 
dorsal surface between the procephalic and caudal regions, and
 
therefore necessarily separates the caudal and procephalic lobes ;
 
but, since the ventral plate does not become shortened in the
 
process, and the embryo cannot straighten itself in the egg-shell,
 
it necessarily becomes ventrally flexed.
 
 
 
If there were but little food yolk this flexure would naturally
 
 
 
 
 
 
 
TRACHEATA. 443
 
 
 
 
 
 
 
cause the whole embryo to be bent in so as to have the ventral
 
surface concave. But instead of this the flexure is at first confined to the two bands which form the ventral plate. These
 
bands, as shewn in fig. 201 A, acquire a true ventral flexure, but
 
the yolk forms a projection a kind of yolk sack as Barrois
 
(No. 441) calls it distending the thin integument between the
 
two ventral bands. This yolk sack is shewn in surface view in
 
 
 
 
 
 
 
 
 
FlG. 201. TWO LATE STAGES IN THE DEVELOPMENT OF AGELENA LABYRINTHICA.
 
 
 
A. Embryo from the side at the stage when there is a large ventral protuberance
 
of yolk. The angle between the line of insertion of the permanent and provisional
 
appendages shews the extent of the ventral flexure.
 
 
 
B. Embryo nearly ready to be hatched. The abdomen which has not quite
 
acquired its permanent form is seen to be pressed against the ventral side of the
 
thorax.
 
 
 
prJ. procephalic lobe; pd. pedipalpi ; ch. chelicerae ; c,L caudal lobe; pr.p. provisional appendages.
 
 
 
fig. 20 1 A and in section in fig. 206. At a later period, when
 
the yolk has become largely absorbed, the true nature of the
 
ventral flexure becomes quite obvious, since the abdomen of the
 
young Spider, while still in the egg, is found to be bent over so
 
as to press against the ventral surface of the thorax (fig. 201 B).
 
The general character of the changes which take place
 
during this period in the development is shewn in fig. 201 A and
 
B representing two stages in it. In the first of these stages
 
there is no constriction between the future thorax and abdomen.
 
 
 
 
 
 
 
444 ACARINA.
 
 
 
 
 
 
 
The four pairs of provisional appendages exhibit no signs of
 
atrophy ; and the extent of the ventral flexure is shewn by the
 
angle formed between the line of their insertion and that of the
 
appendages in front. The yolk has enormously distended the
 
integument between the two halves of the ventral plate, as is
 
illustrated by the fact that, at a somewhat earlier stage than
 
that figured, the limbs cross each other in the median ventral
 
line, while at this stage they do not nearly meet The limbs
 
have acquired their full complement of joints, and the pedipalpi
 
bear a cutting blade on their basal joint.
 
 
 
The dorsal surface between the prominent caudal lobe and
 
the procephalic lobes forms more than a semicircle. The terga
 
are fully established, and the boundaries between them, especially
 
in the abdomen, are indicated by transverse markings. A large
 
lower lip now bounds the stomodaeum, and the upper lip has
 
somewhat atrophied. In the later stage (fig. 201 B) the greater
 
part of the yolk has passed into the abdomen, which is now to
 
some extent constricted off from the cephalo-thorax. The
 
appendages of the four anterior abdominal somites have disappeared, and the caudal lobe has become very small. In front
 
of it are placed two pairs of spinning mammillae. A delicate
 
cuticle has become established, which is very soon moulted.
 
 
 
Acarina. The development of the Acarina, which has been mainly
 
investigated by Claparede (No. 446), is chiefly remarkable from the frequent
 
occurrence of several larval forms following each other after successive
 
ecdyses. The segmentation (vide p. 116) ends in the formation of a blastoderm of a single layer of cells enclosing a central yolk mass.
 
 
 
A ventral plate is soon formed as a thickening of the blastoderm, in which
 
an indistinct segmentation becomes early observable. In Myobia, which is
 
parasitic on the common mouse, the ventral plate becomes divided by five
 
constrictions into six segments (fig. 202 A), from the five anterior of which
 
paired appendages very soon grow out (fig. 202 B) The appendages are the
 
chelicerae (ch} and pedipalpi (pd] and the first three pairs of limbs (p^fi 1 }.
 
On the dorsal side of the chelicerae a thickened prominence of the ventral
 
plate appears to correspond to the procephalic lobes of other Arachnida.
 
The part of the body behind the five primitive appendage-bearing segments
 
appears to become divided into at least two segments. In other mites the
 
same appendages are formed as in Myobia, but the preceding segmentation
 
of the ventral plate is not always very obvious.
 
 
 
In Myobia two moultings take place while the embryo is still within the
 
primitive egg-shell. The first of these is accompanied by the apparently
 
total disappearance of the three pediform appendages, and the complete
 
 
 
 
 
 
 
TRACK EAT A.
 
 
 
 
 
 
 
445
 
 
 
 
 
 
 
coalescence of the two gnathiform appendages into a proboscis (fig. 202 C).
 
The feet next grow out again, and a second ecdysis then takes place. The
 
embryo becomes thus inclosed within three successive membranes, viz. the
 
original egg-shell and two cuticular membranes (fig. 202 D). After the
 
second ecdysis the appendages assume their final form, and the embryo
 
leaves the egg as an hexapodous larva. The fourth pair of appendages is
 
 
 
 
 
 
 
 
 
FIG. 202. FOUR SUCCESSIVE STAGES IN THE DEVELOPMENT OF MYOBIA MUSCULI.
 
(After Claparede.)
 
 
 
J 1 j 4 . post-oral segments ; ch. chelicerae ; pd. pedipalpi ; pr. proboscis formed by
 
the coalescence of the chelicerse and pedipalpi ; p l , /*, etc. ambulatory appendages.
 
 
 
acquired by a post-embryonic metamorphosis. From the proboscis are
 
formed the rudimentary palpi of the second pair of appendages, and two
 
elongated needles representing the chelicerae.
 
 
 
In the cheese mite (Tyroglyphus) the embryo has two ecdyses which are
 
not accompanied by the peculiar changes observable in Myobia : the
 
cheliceras and pedipalpi fuse however to form the proboscis. The first
 
larval form is hexapodous, and the last pair of appendages is formed at a
 
subsequent ecdysis.
 
 
 
In Atax Bonzi, a form parasitic on Unio, the development and metamorphosis are even more complicated than in Myobia. The first ecdysis
 
occurs before the formation of the limbs, and shortly after the ventral plate
 
has become divided into segments. Within the cuticular membrane resulting
 
from the first ecdysis the anterior five pairs of limbs spring out in the usual
 
fashion. They undergo considerable differentiation ; the chelicerae and
 
pedipalpi approaching each other at the anterior extremity of the body, and
 
the three ambulatory legs becoming segmented and clawed. An oesophagus,
 
a stomach, and an oesophageal nerve-ring are also formed. When the larva
 
 
 
 
 
 
 
446 ACARINA.
 
 
 
 
 
 
 
has attained this stage the original egg-shell is split into two valves and
 
eventually cast off, but the embryo remains enclosed within the cuticular
 
membrane shed at the first ecdysis. This cuticular membrane is spoken of
 
by Claparede as the deutovum. In the deutovum the embryo undergoes
 
further changes ; the chelicerae and pedipalpi coalesce and form the
 
proboscis ; a spacious body cavity with blood corpuscles appears ; and the
 
alimentary canal enclosing the yolk is formed.
 
 
 
The larva now begins to move, the cuticular membrane enclosing it is
 
ruptured, and the larva becomes free. It does not long remain active, but
 
soon bores its way into the gills of its host, undergoes a fresh moult, and
 
becomes quiescent. The cuticular membrane of the moult just effected
 
swells up by the absorption of water and becomes spherical. Peculiar
 
changes take place in the tissues, and the limbs become, as in Myobia,
 
nearly absorbed, remaining however as small knobs. The larva swims
 
about as a spherical body within its shell. The feet next grow out afresh,
 
and the posterior pair is added. From the proboscis the palpi (of the
 
pedipalpi) grow out below. The larva again becomes free, and amongst
 
other changes the chelicerae grow out from the proboscis. A further ecdysis,
 
with a period of quiescence, intervenes between this second larval form and
 
the adult state.
 
 
 
The changes in the appendages which appear common to the Mites
 
generally are (i) the late development of the fourth pair of appendages, which
 
results in the constant occurrence of an hexapodous larva ; and (2) the early
 
fusion of the chelicerae and pedipalpi to form a proboscis in which no trace
 
of the original appendages can be discerned. In most instances palpi and
 
stilets of variable form are subsequently developed in connexion with the
 
proboscis, and, as indicated in the above descriptions, are assumed to correspond with the two original embryonic appendages.
 
 
 
TJie history of tJie germinal layers.
 
 
 
It is a somewhat remarkable fact that each of the groups of
 
the Arachnida so far studied has a different form of segmentation. The types of Chelifer and the Spiders are simple modifications of the centrolecithal type, while that of Scorpio, though
 
apparently meroblastic, is probably to be regarded in the same
 
light (vide p. 120 and p. 434). The early development begins in
 
the Scorpion and Spiders with the formation of a ventral plate,
 
and there can be but little doubt that Chelifer is provided
 
with an homologous structure, though very probably modified,
 
owing to the small amount of food-yolk and early period of
 
hatching.
 
 
 
The history of the layers and their conversion into the organs
 
has been studied in the case of the Scorpion (Metschnikoff, No.
 
 
 
 
 
 
 
TRACHEATA. 447
 
 
 
 
 
 
 
434), and of the Spiders ; and a close agreement has been found
 
to obtain between them.
 
 
 
It will be convenient to take the latter group as type, and
 
simply to call attention to any points in which the two groups
 
differ.
 
 
 
The epiblast. The epiblast, besides giving rise to the skin
 
(hypodermis and cuticle), also supplies the elements for the
 
nervous system and organs of sense, and for the respiratory
 
sacks, the stomodaeum and proctodaeum.
 
 
 
At the period when the mesoblast is definitely established,
 
the epiblast is formed of a single layer of columnar cells in the
 
region of the ventral plate, and of a layer of flat cells over other
 
parts of the yolk.
 
 
 
When about six segments are present the first changes take
 
place. The epiblast of the ventral plate then becomes somewhat
 
thinner in the median line than at the two sides (fig. 203 B). In
 
succeeding stages the contrast between the median and the
 
lateral parts becomes still more marked, so that the epiblast
 
becomes finally constituted of two lateral thickened bands, which
 
meet in front in the procephalic lobes, and behind in the caudal
 
lobe, and are elsewhere connected by a very thin layer (fig.
 
203 C). Shortly after the appendages begin to be formed, the
 
first rudiments of the ventral nerve-cord become established as
 
epiblastic thickenings on the inner side of each of the lateral
 
bands. The thickenings of the epiblast of the two sides are
 
quite independent, as may be seen in fig. 203 C, vn, taken from a
 
stage somewhat subsequent to their first appearance. They are
 
developed from before backwards, but either from the first, or in
 
any case very soon afterwards, cease to form uniform thickenings,
 
but constitute a linear series of swellings the future ganglia
 
connected by very short less prominent thickenings of the epiblast (fig. 200 C). The rudiments of the ventral nerve-cord are
 
for a long time continuous with the epiblast, but shortly after the
 
establishment of the dorsal surface of the embryo they become
 
separated from the epiblast and constitute two independent
 
cords, the histological structure of which is the same as in other
 
Tracheata (fig. 206, vn\
 
 
 
The ventral cords are at first composed of as many ganglia
 
as there are segments. The foremost pair, belonging to the
 
 
 
 
 
 
 
448
 
 
 
 
 
 
 
ARACHNIDA.
 
 
 
 
 
 
 
segment of the chelicerae, lie immediately behind the stomodaeum,
 
and are as independent of each other as the remaining ganglia.
 
Anteriorly they border on the supra-cesophageal ganglia. When
 
the yolk sack is formed in connection with the ventral flexure of
 
the embryo, the two nerve-cords become very widely separated
 
(fig. 206, vn) in their middle region. At a later period, at the
 
stage represented in fig. 201 B, they again become approximated
 
in the ventral line, and delicate commissures are formed uniting
 
 
 
 
 
 
 
 
 
 
 
FIG. 203. TRANSVERSE SECTIONS THROUGH THE VENTRAL PLATE OF AGELBNA
 
LABYRINTHICA AT THREE STAGES.
 
 
 
A. Stage when about three segments are formed. The mesoblastic plate is not
 
divided into two bands.
 
 
 
B. Stage when six segments are present (fig. ?oo B). The mesoblast is now
 
divided into two bands.
 
 
 
C. Stage represented in fig. 200 D. The ventral cords have begun to be formed
 
on thickenings of the epiblast, and the limbs are established.
 
 
 
ep. epiblast ; me. mesoblast ; me.s. mesoblastic somite ; 7>n. ventral nerve-cord ;
 
yk. yolk.
 
 
 
the ganglia of the two sides, but there is no trace at this or any
 
other period of a median invagination of epiblast between the
 
two cords, such as Hatschek and other observers have attempted
 
to establish for various Arthropoda and Chaetopoda. At the
 
stage represented in fig. 201 A the nerve ganglia are still present
 
in the abdomen, though only about four ganglia can be distinguished. At a later stage these ganglia fuse into two continuous
 
 
 
 
 
 
 
TRACHEATA. 449
 
 
 
 
 
 
 
cords, united however by commissures corresponding with the
 
previous ganglia.
 
 
 
The ganglia of the chelicerae have, by the stage represented
 
in fig. 20 1 B, completely fused with the supra-oesophageal ganglia
 
and form part of the oesophageal commissure. The cesophageal
 
commissure is however completed ventrally by the ganglia of
 
the pedipalpi.
 
 
 
The supra-cesophageal ganglia are formed independently of
 
the ventral cords as two thickenings of the procephalic lobes (fig.
 
205). The thickenings of the two lobes are independent, and
 
each of them becomes early marked out by a semicircular groove
 
(fig. 200 D) running outwards from the upper lip. Each thickening eventually becomes detached from the superficial epiblast,
 
but before this takes place the two grooves become deeper,
 
and on the separation of the ganglia from the epiblast, the
 
cells lining the grooves become involuted and detached from
 
the skin, and form an integral part of the supra-oesophageal
 
ganglia.
 
 
 
At the stage represented in fig. 201 B the supra-oesophageal ganglia
 
are completely detached from the epiblast, and are constituted of the
 
following parts : (i) A dorsal section formed of two hemispherical lobes,
 
mainly formed of the invaginated lining of the semicircular grooves. The
 
original lumen of the groove is still present on the outer side of these
 
lobes. (2) Two central masses, one for each ganglion, formed of punctiform tissue, and connected by a transverse commissure. (3) A ventral
 
anterior lobe. (4) The original ganglia of the chelicerae, which form the
 
ventral parts of the ganglia 1 .
 
 
 
The later stages in the development of the nervous system have not
 
been worked out.
 
 
 
The development of the nervous system in the Scorpion is almost
 
identical with that in Spiders, but Metschnikoff believes, though without
 
adducing satisfactory evidence, that the median integument between the
 
two nerve cords assists in forming the ventral nerve cord. Grooves are
 
present in the supra-cesophageal ganglia similar to those in Spiders.
 
 
 
The mesoblast. The history of the mesoblast, up to the
 
formation of a ventral plate subjacent to the thickened plate of
 
epiblast, has been already given. The ventral plate is shewn
 
in fig. 203 A. It is seen to be formed mainly of small cells,
 
 
 
1 For further details vide self, No. 440.
 
B. II. 29
 
 
 
 
 
 
 
45O ARACHNIDA.
 
 
 
 
 
 
 
but some large cells are shewn in the act of passing into it
 
from the yolk. During a considerable section of the subsequent development the mesoblast is confined to the ventral
 
plate.
 
 
 
The first important change takes place when about six
 
somites are established ; the mesoblast then becomes divided