The Works of Francis Balfour 2-7

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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.

The Ovum and Spermatozoon | The Maturation and Impregnation of the Ovum | The Segmentation of the Ovum | Dicyemae and Orthonectidae Dicyema | Porifera | Coelenterata | Platyhelminthes | Rotifera | Mollusca | Polyzoa | Brachiopoda | Chilopoda | Discophora | Gephyrea | Chaetognatha | Nemathelminthes | Tracheata | Crustacea | Pcecilopoda | Echinodermata | Enteropneusta | Bibliography
Online Editor 
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This historic 1885 book edited by Foster and Sedgwick is the second of Francis Balfour's collected works published in four editions. Francis (Frank) Maitland Balfour, known as F. M. Balfour, (November 10, 1851 - July 19, 1882) was a British biologist who co-authored embryology textbooks.

The Works of Francis Balfour Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.

Modern Notes:

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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

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Vol II. A Treatise on Comparative Embryology (1885)


ALTHOUGH there is perhaps no group in the animal kingdom the ontogeny of which would better repay a thorough investigagation than the Turbellarians, yet the difficulties to be overcome have hitherto proved too great.

The fresh-water Rhabdocoela and Dendroccela do not undergo any metamorphosis, and leave the ovum in a condition in which they cannot easily be distinguished in their general appearance from Infusoria. Many marine Dendroccela also develop directly, while, as was first shewn by Joh. Miiller, other marine Dendroccela undergo a more or less complicated metamorphosis.

Marine Dendroccela. Of the marine Dendroccela which do not undergo a metamorphosis the form most fully worked out is Leptoplana tremellaris (vide Keferstein, No. 187, and Hallez, No. 185).

The ova are surrounded by large albuminous capsules secreted by a special gland. They are laid a great number at a

1 I. Turbellaria.

1. Dendrocoela.

2. Rhabdocoela.

II. Nemertea.

1. Anopla.

2. Enopla.

Hi. Trematoda.

1. Distomese.

2. Polystomeae.

iv. Cestoda,


time, and adhere together so as to form masses not unlike the spawn of nudibranchiate Molluscs.

Within the egg-capsule the ovum floats freely and undergoes a segmentation similar in many respects to the characteristic molluscan type. The ovum divides into two, and then into four parts, from each of which a small segment is then separated off. The four small segments, which appear to give rise to the epiblast, increase in number by division and gradually envelop the large segments 1 ; so that an epibolic invagination clearly takes place. Between the small and the large cells is a small segmentation cavity, fig. 86 A and B. At the time when twelve epiblast cells are present, each of the four large cells divides into two unequal parts (Hallez), fig. 86 A. In this way four large (Jiy) and four small cells (;) are formed. The latter are placed at the opposite pole of the ovum to the epiblast cells, and give rise to the mesoblast, while the four large cells remain as the hypoblast.

In the course of the enclosure of the hypoblast cells by the


ep. epiblast; m. mesoblast; hy. yolk cells (hypoblast); bl. blastopore.

epiblast, the mesoblast cells gradually travel towards the formative pole (fig. 86 B). In the process they become first of all divided so as to form four linear streaks, and finally unite into a continuous layer between the epiblast and hypoblast, which obliterates the segmentation cavity (fig. 86 C, m).

Before the completion of the epibole a closely packed layer of fine cilia appears, which causes a rotation of the embryo within the egg-capsule. During the above changes a fifth hypoblast cell is formed by the division of one of those already present ; and at a later period four of the hypoblast cells give rise within

1 It is probable, though it has not been observed, that the growth of the layer of small rclls is a^isted by the formation of fresh cells from the hypoblast spheres.


the nearly closed blastoporic area to four small cells. In connection with these cells a complete hyploblastic wall becomes subsequently established, which encloses the original large hypoblast cells. The latter then become resolved into a vitelline mass.

From a comparison with other types it may be regarded as probable that the enteric wall originates by a process of continuous budding off of small cells from the large cells, which commences with the formation of the four cells above mentioned.

The blastopore becomes nearly obliterated, but whether it gives rise to the mouth, which is formed in the same place, has not been determined. In front of the mouth a small and very transitory rudiment of an upper lip makes its appearance. The protrusible pharynx is stated by Hallez to arise as an hypoblastic bud, while its sheath has an epiblastic origin. Two pairs of eyes and the supra-cesophageal ganglia also become early developed.

The peripheral ciliated layer of small cells becomes divided into two strata, of which the outer remains ciliated and forms the true epiblast : the inner probably forms the cutis. In it are developed rod-like bodies, which seem to be homologous with the thread cells of the Ccelenterata, so that if the views put forward in the previous chapter as to the similarity of the turbellarian and ccelenterate larvae are correct, the cutis corresponds with the deeper layer of the ccelenterate epiblast. The mesoblast, like the epiblast, becomes divided into two strata. The outer one is stated to form the circular and longitudinal muscles; the inner one to give rise to a muscular reticulum, the spaces within which constitute the parenchymatous body cavity.

The later changes are not of great importance. At a period slightly after the formation of the mouth and ganglia two pairs of stiff hairs become formed at the sides of the body. The embryo has by this time grown so as to fill up its capsule, in which however it continues rapidly to rotate, and also commences to exhibit active contractions. It next becomes hatched, and passes from a spherical to a flattened elongated form. The ventral oral opening is at first central, but soon, by a process of unequal growth, becomes carried towards the posterior end of the body. The pairs of stiff hairs in the meantime considerably increase in number. The remains of the yolk cells now disappear, and the enteric walls become more distinct. The alimentary canal, which is at first simple in outline like that of a rhabdoccelous Turbel



larian, soon assumes a dendritic form. The young animal after these changes resembles its parent, except in the possession of only two pairs of eyes and in the absence of generative organs.

Of the types with a complete metamorphosis the free larvae of various species of Thysanozoon have been observed by Joh. Miiller (190) and Moseley (189), and the complete development of Eurylepta auriculata has been studied by Hallez.

The stages within the egg of this latter type agree precisely with those already described in Leptoplana. After the formation of the mouth the body elongates, remaining however cylindrical. A fold forms on the anterior side of the mouth, giving rise to a large upper lip. Two posterior processes FIG. 87. LARVA OF EURYLEPTA


are next formed, and other pro- HATCHING.

cesses soon arise, constituting the SID ^ jJ

whole of those found in the free

larva. The embryo next shakes off its egg membranes by a

series of vigorous contractions. When free it has the form repre sented in the annexed figure (fig. 87).

It is so similar to Miiller's (fig. 88) and Moseley's larvae that all three may be dealt with together.

The body is somewhat oval, and slightly pointed behind. At the anterior end are placed the eyes, two in the youngest larva of Miiller, and twelve in the older larva (fig. 88), and in the middle of the ventral surface is the mouth. It is surrounded by a strong fold, and leads into an alimentary canal, which is at first simple, but in the older larvae is much branched. A bilobed ganglion connected with two nerve cords is placed anteriorly. The superficial epithelium is ciliated, and below it is a layer of cells (cutis) derived from the primitive epiblast, in which are formed the usual rods (Hallez). The chief peculiarity of the larva consists in the presence of elongated processes covered with long cilia, and so connected together by a ciliated band that the whole together forms, in Miiller's larva at any rate, a




lobed prceoral ciliated band (fig. 88). This band is not quite so clear in Hallez' figures. Miiller's youngest larva was provided with eight very long lobes; three were dorsal, viz. a median anterior, and two lateral placed far back ; three ventral, viz. a median in the front of the mouth forming a large upper lip, and two processes at the sides of the mouth. The number was completed by two lateral processes of the body. All the processes except the dorsal median one are shewn in fig. 88. In Hallez' larva, fig. 87, the six posterior processes form a rather definite ring, while one flagellum projects from the front end of the body immediately below the eyes, and a second flagellum behind. In Moseley's youngest larva six, processes only were present, though subsequently eight became formed as in Miiller's larvae.

The metamorphosis consists in the whole animal growing longer and flatter, and in the arms becoming gradually shorter and shorter till they finally disappear altogether, and the larva acquires the ordinary adult form.

The lobed larval form of the Turbellaria has some points of resemblance to the Pilidium form of nemertine larva described below, yet its resemblance to this interesting larva is less close than would appear to be the case with certain turbellarian larval forms recently described by Gotte and Metschnikoff, which are in some respects intermediate in character between the larva of Leptoplana and those just described.

The observations of Gotte (No. 184) were made on Planaria Neapolitana and Thysanozoon Diesingi, and those of Metschnikoff (No. 188) on Stylochopsis ponticus. The larvae of all these forms undergo more or less of a metamorphosis, but the accounts of their development are not easily reconciled 1 . The early stages of Planaria are like those of Leptoplana, as

1 The account of Metschnikoff s observations on Stylochopsis ponticus given in the German abstract is too obscure to be placed in the text, but the following are the more important points which can be gleaned from it.

The ovum becomes first divided into eight segments. By further division along the equatorial zone, a ring of small cells is formed which becomes the epiblast. The

B. II. 13


The ciliated band is represented by the black line.

m. mouth ; ./. upper lip.




ANGULATA). (From Agassi z.)

described by Keferstein. Four large hypoblast cells become surrounded by small epiblast cells, which commence to be formed on the dorsal side. The hypoblast cells divide and arrange themselves in two bilaterally-symmetrical rows. A small blastopore is left by the small cells on the ventral surface, which communicates with an otherwise closed and ciliated cavity which is formed between the two rows of hypoblast cells. The blastopore would seem to remain permanently open, and to be placed at the base of a deep pit, lined by epiblast cells, which constitutes the stomodaeum.

The embryo now becomes dorsally convex, while the ventral surface becomes marked with a median furrow and grows out laterally into two lobes, and anteriorly into a ventrally-directed upper lip. The whole surface becomes ciliated, and the cilia are especially prominent on the ventral processes and the summit of the dorsal dome. A bunch of strong cilia becomes formed in front of the dome, and a less marked bunch behind. The larva is now stated by Gotte closely to resemble a Pilidium. It soon, however, extends itself, and the two bunches of cilia become situated at the anterior and posterior extremities of the body. The ventral processes become inconspicuous prominences of the side of the body. Gotte believes that the larva undergoes no further metamorphosis.

A type of Planarian larva (figs. 89 and 90) possibly Plan, angulata, observed by Alex. Agassiz (No. 181), is very different from any other so far described, and is remarkable for being divided into a series of segments corresponding in number with the diverticula of the digestive cavity. In the youngest specimen (fig. 89) the body was nearly cylindrical, and divided into eleven rings, corresponding with as many digestive diverticula. Two eye-spots were present. In a later stage two poles are at this time formed of large cells. At one pole four small cells appear, which are compared by Metschnikoff to the pole cells of the Diptera (vide Chapter on the development of the Insecta). At the opposite pole a blastopore is formed leading into a small segmentation cavity. The epiblast also now gradually grows over the large cells. At the blastopore pole the large cells give rise to the hypoblast and the small cells at the opposite pole assist in forming the epiblast. The blastopore dtappeUB, and with it the segmentation cavity, while the hypoblast, forming a solid mass, becomes divided into two halves (Cf. Planaria Neapolitana). The embryo becomes ciliated and begins to rotate; and the eyes, and somewhat later (?) the nervous ganglion make their appearance.

In the interior a wide cavity develops between the hypoblast cells, which becomes ciliated and is placed in communication with the exterior by an invaginated stomadseum which forms the pharynx.

The larva now, as in Planaria Neapolitana, takes on a Pilidium-like form. Lateral I- >!(.> and an anterior lip grow out from the under surface, and become covered with long cilia, while at the upper pole a long flagellum makes its appearance.



(fig. 90) the body was considerably flattened and had approached more to the planarian form.

If Agassiz' interesting observations can be trusted we have in this larva indications of a distinct segmentation, which are of some morphological importance, especially when taken in connection with the traces of segmentation found amongst the Nemertines.

A further type, with an incomplete metamorphosis, has been observed by Girard (183). It is remarkable for having an uniform.segmentation, and for presenting a quiescent stage after passing through a free larval condition with a large upper lip.

Fresh-water Dendroccela. The development of the freshwater Dendrocoela has been especially investigated by Knappert (No. 186) and Metschnikoff (No. 188).

The ova are very delicate minute naked cells, which to the number of 4 6 or more become enveloped in a capsule or cocoon together with a large mass of yolk cells derived from the vitellarium. The yolk cells exhibit peristaltic movements and send out amoeboid processes. Each ovum when laid becomes surrounded by an extremely delicate membrane, which disappears during the course of development. The capsules consist of a spherical case and a stalk. The latter is first emitted from the female opening as a thread-like body. Its free end becomes attached, and then the remainder of the capsule is ejected.

Impregnation takes place before the formation of the capsule. The segmentation is complete. The ovum first divides into two segments. One of these next divides, forming three segments. There are subsequently stages with four, eight, sixteen, and thirty-two segments.

Metschnikoff's results on the stages subsequent to the segmentation are not in complete harmony with those of Knappert ; but no doubt represent an advance in our knowledge, and I shall follow them here. His observations were made on Planaria polychroa.

In the earliest stage observed by him the segmentation was already far advanced, but no membrane was present round the ovum. At a later stage the ovum becomes more or less bell-shaped or hemispherical, and encloses within its concavity a mass of yolk elements. It is now formed of three concentric layers. An outer layer of flattened cells the epiblast, a middle layer of fused cells the mesoblast, and an inner solid mass of yolk cells the hypoblast.

At the upper pole is formed the protrusible pharynx (cf. Knappert), which is provided with a provisional musculature and a lumen. By its contractions it takes up the yolk elements which surround the embryo, and the rapid growth of the embryo no doubt takes place at their expense. The embryo



gradually loses its hemispherical form, and assumes an elongated and flattened shape. It acquires a coating of cilia by means of which it rotates. On the fifth day it is hatched.

The alimentary tract long remains solid, even after it has acquired its branched form. The pharynx becomes withdrawn as soon as the larva is hatched. It loses its provisional muscles, and subsequently acquires a permanent musculature. The young after hatching attach themselves to the body of their parent, on which they feed (?).

Rhabdoccela. The development of some of the Rhabdoccela has recently been studied by Hallez. The ova are mostly laid in capsules, one in each capsule. Sometimes the development commences before the capsules are laid, at other times not till afterwards. In certain forms (Mesostomum) there are summer eggs with thin capsules which develop within the parent, while hard capsules, forming what are known as winter eggs, are laid in the autumn, and the embryo hatched in the spring.

The ova of the Rhabdoccela like those of the fresh-water Dendroccela are enveloped in yolk elements derived from the vitellarium.

The segmentation probably takes place in the same way as in Leptoplana. A stage with four equal cells has been observed by Hallez, and there is subsequently an epibolic gastrula. The embryo becomes ciliated while still within the capsule and, according to Hallez, the pharynx arises as a bud of the hypoblast. The proboscis in Prostomum originates as an epiblastic invagination.


Some Nemertea develop without and some with a metamorphosis.

The most remarkable type of Nemertine development with a metamorphosis is that in which the ovum develops into a peculiar larval form known as Pi lid ium, within which the perfect worm is subsequently evolved. Closely allied to this type is one in which the sexual worm is developed within a larval form as in 1'ilidium, but in which the larva has no free swimming stage, and is therefore without the characteristic appendages of the Pilidium. This is known as the type of Desor and is confined (?) to the genus Lineus. The Pilidium and the Desor type may be first considered (vide Barrois, No. 192).

The type of Desor. The segmentation is regular and leads to the formation of a blastosphere with a large segmentation


cavity. The blastosphere is converted by invagination into a gastrula (fig. 91 A). The blastopore is soon carried relatively


A is a side view in optical section.

B and C are two later stages from the ventral (oral) surface.

ae. archenteron ; sc. segmentation cavity ; hy. hypoblast ; me. mesoblast ; ep. epiblast ; m. mouth ; st. stomach ; pr. d. prostomial disc ; po. d. metastomial disc ; pr. proboscis.

forwards by the elongation backwards of the archenteron, and, according to Barrois, actually forms the mouth. Owing to the elongation of the archenteric cavity the embryo assumes a bilateral form (fig. 92 A) in which the dorsal and ventral surfaces can be distinguished, the mouth (m.) being situated on the ventral surface.

Immediately after the completion of the gastrula a remarkable series of phenomena takes place. The embryo when viewed from the ventral surface assumes a pentagonal form (fig. 91 B), and four invaginations of the epiblast make their appearance on the ventral surface (fig. 92 A), two in front of {pr. d.) and two behind {po. d.) the mouth ; they result in the formation of four thickened discs. These discs soon become separated from the external skin, which closes in forming an unbroken layer over them (fig. 91 C). The discs grow rapidly, and first the prostomial pair and subsequently the metastomial fuse together, and finally the whole four unite into a continuous ventral plate ; analogous it would seem to the ventral plate of chsetopodan and



arthropodan embryos. The plate so formed gradually extends itself so as to close over the dorsal surface, and to form a complete skin within the primitive larval skin, which at this period is richly ciliated, though the embryo is not yet hatched


A. Side view of an embryo at a very early stage as an opaque object.

B and C. Two late stages, seen as transparent objects from the ventral surface.

at. archenteron; m. mouth; pr, d. prostomial disc; po.d. metastomial disc; cs. lateral pit developing in B as a diverticulum from the oesophagus; pr. proboscis ; ms. muscular layer (?); Is. larval skin about to be thrown off; me. mesoblast; st. stomach.

(fig. 91 C). While these changes are taking place, there are budded off from the invaginated discs a number of fatty cells, which fill up the space between the discs and the archenteron, and eventually form the mesoblastic reticulum. During this stage the rudiment of the proboscis also makes its appearance as a solid process of epiblast, which grows backwards from the point of fusion of the two prostomial discs at the front end of the embryo (fig. 91 C, pr.). A lumen is excavated in it at a later period. The lateral organs or cephalic pits arise in a somewhat unexpected fashion as a pair of diverticula from the


oesophagus (fig. 92 B, cs.) 1 , which soon fuse with the walls of the body at the junction of the prostomial and metastomial plates (fig. 92 C, cs.), although they remain for some time attached to the oesophagus by a solid cord.

During these changes the original larval skin separates itself from the subjacent layer formed by the discs (fig. 92, B and C), and is soon thrown off completely, leaving the already ciliated (fig. 92 C) external layer of the invaginated discs as the external skin of the young Nemertine. During, and subsequently to, the casting off of the embryonic skin, important changes take place in the constitution of the various layers of the body, resulting in the formation of the vascular system and other mesoblastic organs, the nervous system, and the permanent alimentary tract. These changes appear to me to stand in need of further elucidation ; and the account below must be received with a certain amount of caution.

It has been already stated that the two discs give rise to fatty cells, which occupy the space between the walls of the body and the archenteron. At the period of the casting off of the embryonic skin fresh changes take place. The discs become very much thickened, and then divide into two layers, which become the epidermis and subjacent muscular layers. The muscular layers arise in two masses, separated by the two cephalic sacks. The anterior mass is formed as an unpaired anterior thickening, followed by two lateral thickenings. The posterior mass is much thinner, in correspondence with the rapid elongation of the metastomial portion of the embryo.

The cells originally split off from the discs undergo considerable changes, some of them arrange themselves around the proboscis as a definite membrane, which becomes the proboscidean sheath, some also form a true splanchnic layer of mesoblast, and the remainder, which are especially concentrated during early embryonic life in the anterior parts of the body, form the general interstitial connective tissue. The cephalic ganglia are stated to become gradually differentiated in the prostomial mesoblast, and the two cords connected with them in the metastomial mesoblast.

At the time when the larval skin is cast off the original mouth becomes closed, and it is not till some time afterwards that a permanent mouth is formed in the same situation. During the early part of embryonic life the intestine is lined with columnar cells, but, before the loss of the larval skin, the walls of the intestine undergo a peculiar metamorphosis. Their cells either fuse or become indistinguishable, and their protoplasm appears to become converted into yolk-spherules, which fill up the whole space within

1 Biitschli for Pilidium regards these pits as formed by imaginations of the epiblast, but Metschnikoff s statements are in accordance with those in the text.


the walls of the body, and are only prevented from extending forwards by a membrane of connective tissue. This mass gradually forms itself into a distinct canal, lined by columnar cells.

Pilidium. In the case of the true Piltdium type, the larva is hatched very early and leads the usual existence of surface larvae. A regular segmentation is followed by an invagination which does not however cause the complete obliteration of the segmentation cavity (fig. 93 A, a.e.).

The primitive alimentary tract so formed becomes divided into cesophageal and gastric regions (fig. 93 B, oe. and .$/.). Even while the invagination of the archenteron is proceeding, the larva becomes ciliated throughout, and assumes a somewhat conical form, the apex of the cone being opposite the flat ventral surface on which the mouth is situated (fig. 93, A and B). From

FIG. 93. Two STAGES IN THE DEVELOPMENT OF PILIDIUM. (After Metschnikoff.) of. archenteron ; <r. oesophagus ; st. stomach ; am. amnion ; pr.d. prostomial disc ; PO. d. metastomial disc; c.s. cephalic sack.

the apex a flagellum projects in many forms, giving the larva a helmet-like appearance. In other forms a bunch of long cilia takes the place of the flagellum (fig. 94), and in others again the flagellum is not represented. After the completion of the invagination a lobe grows out on each side of the mouth, and less well developed lobes may appear anteriorly and posteriorly. Round the edge of the ventral surface a ciliated band makes its appearance.



Two pairs of imaginations of the skin, just as in the type of Desor, now make their appearance, one pair in front of and the other behind the mouth (fig. 93 B, pr.d. and po.d.}, and each of them by the closure of the opening of invagination forms a sack, the outer wall of which becomes very thin and the inner wall (corresponding with the whole invagination of the type of Desor) very thick. The inner walls of the four thickenings, which I may speak of as discs, now fuse together, each disc first uniting with its fellow, and finally the two pairs uniting.

A ventral germinal plate is thus established, which gradually grows round the intestine of the Pilidium to form the skin of the future Nemertine. The outer thin layer of each of the discs grows part passu with the inner layer, and furnishes an amnion-like covering for the embryo which is forming within the Pilidium (fig. 94, an}.

In connection with the young vermiform Nemertine there is formed on each side an outgrowth from the oesophagus (fig. 94) which is eventually placed in communication with the exterior

FIG. 94.




&. oesophagus ; st. stomach ; i. intestine ; pr. proboscis ; Ip. lateral pit ; an. amnion ; n. nervous system.


by a ciliated canal 1 . The proboscis arises as an hollow invagination at the point where the two anterior discs fuse in front.

When the young Nemertine has become pretty well formed within the Pilidium it becomes ciliated, begins to move, and eventually frees itself and leads an independent existence, leaving its amnion in the Pilidium which continues to live for some time.

The central nervous system (fig. 94) is developed either before or after the detachment of the young Nemertine, according to Metschnikoff as a thickening of the epiblast. The young Nemertine is at first without an anus.

The development of the Nemertine within the Pilidium is clearly identical with that of the Lineus embryo within the larval skin ; the formation of an amnion in the Pilidium constituting the only important difference which can be pointed out between the modes of origin of the young Nemertine in the two types.

So far as is known the forms which develop in a Pilidium, or according to the type of Desor, all belong to the division of the Nemertines without stylets in the proboscis, known as the Anopla.

Development without Metamorphosis. The majority of the Nemertea, including the whole (?) of the Enopla, develop without a metamorphosis. The observations which have been made on this type are not very satisfactory, but appear to indicate that the formation of the hypoblast may take place cither by invagination or by delamination.

Invaginate types have been observed by Barrois (No. 192), Dieck (No. 196) and Hubrecht.

Barrois' fullest observations were made on Amphipoi~us lactifloreus (one of the Enopla), and those of Dieck on Cephalothrix galathece (one of the Anopla).

A regular segmentation is followed by a blastosphere stage with a small segmentation cavity. In Barrois' type the inner ends of the cells of the blastosphere are stated to fuse into a kind of syncytium. A small invagination takes place, and the cells which take part in it separate from the

1 This is the view of both Metschnikoff (No. 202) and Leuckart and Pagcnstecher (No. 201), and is further confirmed by Barrois, but Biitschli (No. 193), though he has not observed the earliest stages of their outgrowth, believes them to be invaginations of the Nemertine skin.


epiblast, and then fuse with the syncytium within the blastosphere. Dieck finds that in Cephalothrix the invaginated mass simply vanishes.

Barrois' statements about the fusion of the syncytium derived from the epiblast cells with the invaginated cells must be regarded as very doubtful. The formation of the germinal layers takes place, according to Barrois, by the separation of the internal mass of cells into mesoblast and hypoblast. The proboscis is formed, according to this author, from the mesoblastic tissues. Dieck, on the other hand, with greater probability, states that the proboscis is formed by an invagination. In Cephalothrix a further point deserves notice, in that the whole of the primitive epiblast becomes shed. In this fact there may perhaps be recognised the last trace of a metamorphosis like that in the type of Desor.

Delaminate types have been studied by Barrois (No. 192) and Hoffman (No. 198), both of whom give circumstantial accounts of their development.

Hoffman's account is especially deserving of attention, since his observations were, to a great extent, made by means of artificial sections. The following account is taken from him. His observations were made on Tetrastemma varicolor, and Tetrastemma appears to be the genus in which this type of development has been most completely made out. After a regular segmentation the embryo forms a solid mass of cells, the outermost of which soon become distinguished as a separate epiblastic layer. At the same time the larva leaves the egg, and the epiblast cells become coated by an uniform covering of cilia. At the anterior extremity of the body is a bunch of long cilia ; and at the hinder end two stiff bristles are formed, but soon disappear.

The internal mass of cells is still quite uniform, but as the larva grows in length the outermost of them arrange themselves as a columnar layer, constituting the mesoblast. Of the cells internal to the mesoblast the outer become columnar, and are converted into the walls of the alimentary tract, while the inner ones undergo fatty degeneration, and form a kind of foodyolk. In the later development the characters of the adult are gradually acquired without metamorphosis, and the larval skin passes directly into that of the adult. Both mouth and anus are formed nearly simultaneously by a rupture of the enteric wall from within. The nervous system arises as a thickening of the epiblast, which Hoffman states he has been able to see in sections. Hoffman also states that the epithelium of the proboscis is formed as a diverticulum of the alimentary tract, and that its sheath is formed by a special mesoblastic growth.

Barrois is less precise than Hoffman, from whom he differs in certain particulars. Hoffman's statements about the proboscis are important if accurate, but require further confirmation.

Malacobdella. The early stages in development of the peculiar ectoparasitic Nemertine Malacobdella have been worked by Hoffman (No. 199) by means of sections, and there appears to be a close agreement between the development of Malacobdella and that of Tetrastemma.

The segmentation is uniform, and there is no trace of a segmentation


cavity. On the third day after impregnation the outermost cells of the embryo become flattened and ciliated, and distinguished from the remaining spherical cells of the embryo as the epiblast. With the appearance of cilia a rotation of the embryo commences. On the fourth day the embryo becomes oval, and at one of the poles the future anal pole a separation takes place between the epiblast and the inner cells, giving rise to the body cavity. In it are a number of loose oval cells, which soon become stellate, and form a mesoblastic reticulum connecting the body-wall nnd central cells of the embryo, which may now be spoken of as hypoblast. The body-cavity increases in size, leaving at last the hypoblast and epiblast united only at one point the oral pole at which, on the fifth day, a crown of long cilia appears. The solid mass of hypoblast in the interior becomes differentiated into an outer layer of cells the true glandular epithelium of the alimentary tract and an inner core, the cells of which soon undergo fatty degeneration, and serve as food-yolk.

The later stages of development, and the formation of the proboscis, etc., have not been worked out.

General considerations. Of the types of larvae hitherto found amongst the Nemertea, those with a metamorphosis, viz. the Pilidium type and that of Desor, are to be regarded as the primitive. But even in Pilidium there are evidences of a great abbreviation in development. Pilidium itself is probably a more or less modified ancestral form, while the peculiar development of the Nemertine within it is to be explained as a very much shortened record of a long series of changes by which the Pilidium became gradually converted into a Nemertine. The formation of the body wall of the Nemertine by four epiblastic invaginations is a remarkable cmbryological phenomenon, for which it is not easy to assign a satisfactory meaning ; and it is probable that it is merely a secondary process of growth similar to the formation of imaginal discs in the larvae of Diptera (vide Chapter on Tracheata), which has had its origin in the abbreviation of the development just alluded to. The development on the type of Desor is clearly a simplification of the Pilidium type, and its peculiarities are to be explained by the fact that the first larval form has no free existence. The types without metamorphosis have no doubt a development of a still more simplified character ; they are remarkable however in presenting us, if the existing descriptions are to be trusted, with examples of delamination and invagination coexisting in closely allied forms.



The eggs of the Trematoda consist of a germ or true ovum enclosed in a mass of yolk cells, which undergo disintegration and subsequent absorption at varying periods of the development. From the observations of E. van Beneden (No. 218) Zeller (No. 217), etc. it is known that the segmentation is usually complete, but generally somewhat irregular.

Unfortunately we are still completely in the dark as to the mode of formation of the germinal layers. The embryos of the entoparasitic forms or Distomeae become free in a very imperfect condition, and the ova are small ; while in the Polystomeae the development is as a rule nearly completed before hatching, and the ova are large. It will be convenient to treat separately the development of the two groups.

Distomeae. The embryos of the Distomeae are hatched either in some moist place or more usually in water. In the majority of genera the larvae pass through a complicated metamorphosis, accompanied by alternations of generations. But for some genera, e.g. Holostomum, etc., the life history has not yet been made out. The whole life history of comparatively few forms has been followed, but sufficient fragments are known to justify us in making certain general statements, which no doubt hold true for a large proportion of the Distomeae.

The larvae are usually ciliated (fig. 95 A), but sometimes naked.

The ciliated forms are generally completely covered with cilia, but in Distommn lanceolatum the cilia are confined to an area at the front end of the body, in the centre of which a median spine is placed. An x shaped pigment spot, sometimes provided with a rudimentary lens (Monostomum mutabile\ is also generally situated on the dorsal surface.

In some intances a more or less completely developed alimentary tract is present (Monostomum capitellum, Amphistomum subclavatum\ but usually there can only be distinguished in the interior of the larva a transparent mass of cells bounded by a more or less distinctly marked body wall with ciliated excretory channels.

Ed. van Beneden has shewn that the ciliated covering is developed while the embryo is still in the egg, and long before the yolk cells are completely absorbed. It would seem that even before hatching this ciliated covering is to a great extent independent of the mass within. In the


larva of Monostomum mutabile (fig. 95 A), which offers an example of an extreme case of the kind, there is present within the ciliated epidermis a fully-developed independent worm.

The non-ciliated larvae are less highly organized than the ciliated forms, and are covered by a cuticle : their anterior extremity is sometimes provided with a circular plate armed with radiate ridges and spines.

The free-swimming or creeping embryos make their way into or on to the body of some invertebrate (occasionally vertebrate) form, usually a Mollusc, to undergo the first stage in their metamorphosis. They may either do this on the gills of their host, or very frequently they bore their way into the interior of the body. Soon after the larvae have reached a satisfactory position the epidermis becomes stripped off, and there emerges a second larval form developed in the interior of the first larva, much as a Nemertine is developed within the larva of Desor. In the case of Monostomum mutabile the new worm is, as stated above, fully formed within the ciliated larva at the time of hatching.

The worm which proceeds from the above metamorphosis has different characters corresponding with those of the larva from which it proceeded. If the original larva had an alimentary canal it has one also, and then grows into the form known as a Redia (Fig. 95, B and C).

The Redia has anteriorly a mouth leading into a muscular pharynx and thence into a caecal stomach. Posteriorly the body is prolonged into a kind of blunt caudal process, at the commencement of which are a pair of lateral papillae. There is a perivisceral cavity, and the body walls are traversed by excretory tubes.

If the original larva is without an alimentary tract, the second form becomes what is known as a Sporocyst. The Sporocyst is a simple elongated sack with a central body cavity ; when derived from the metamorphosis of a ciliated embryo its walls are provided with excretory tubes, but such tubes are absent in Sporocysts developed from non-ciliated larvae. Some Sporocysts send out numerous branches amongst the viscera of their hosts.

The Rediiu and Sporocysts rapidly grow in size and sometimes increase by transverse division. In the course of their



further development one of two things may happen. They may either (i) develop fresh Rediae or Sporocysts by a process of internal budding (fig. 95 C) ; or else (2) there may be formed in them, by an analogous process, larvae with long tails known as Cercariae (fig. 95 D.) The direct development of Cercariae is the usual course, though in Distomum globiparum the reverse is true ; but where this does not take place the Rediae or Sporocysts of the second generation give rise to Cercariae.

The Cercarias are developed from spherical masses of cells found in the body cavity of the Sporocyst or Redia. The exact origin of these masses is still somewhat obscure, but they are stated by Wagener (No. 212) to be derived from the body wall, regarded as internal buds.

The spherical bodies grow rapidly in size, their posterior extremity is prolonged into a process which forms the tail, while the anterior part forms the trunk. When fully formed (fig. 95 E), the trunk has very much the organization of an adult Distomum. There is an anterior and a ventral sucker, the former of which contains the opening of the mouth, and is often provided with a special chitinous armature. The mouth leads into a muscular pharynx, and this into a bilobed caecal alimentary tract. An excretory system of the ordinary type is present, consisting of longitudinal contractile trunks continuous anteriorly with branched ciliated canals, which, as has recently been shewn by Biitschli, may be provided with funnel-shaped ciliated internal openings 1 .


A. Ciliated larva of Monostomum mutabile. a. larval skin. b. Redia developed within it. B. Redia of Monostomum mutabile. C. Redia of Distomum pacificum, with germs of a second brood of Rediae. D. Redia containing Cercariae. E. Cercaria. F. Fullgrown Distomum.

They are probably to be

1 O. Biitschli, "Bemerkungen iib. d. excretorischen Gefassapparat d. Trematoden." Zoologischer Anzeiger, 1879, No. 42.


The contractile trunks unite posteriorly, but instead of opening directly to the exterior are prolonged into a vessel which traverses the substance of the tail, and after a longer or shorter course bifurcates into two branches which open laterally.

The tail is provided with an axial rod of hyaline connective tissue, like the notochord of the tail of a larval Ascidian, and is frequently provided with membranous expansions. It is used as a swimming organ. Beneath the epidermis are layers of circular and longitudinal muscular fibres, the latter arranged in the tail as two bands.

The Cercariae when fully developed leave the Sporocyst or Redia, and then their host, and become free. In most Rediae there is a special opening, not far from the mouth, by which they pass out. There is no such opening in Sporocysts, but the Cercariae bore their way through the walls.

After leaving their parent the Cercariae pass into the external medium, and for a short period have a free existence. They soon however enter a new host, making their w; y into its body by a process of boring, which is effected by the head (especially when armed with chitinous processes) assisted by movements of the tail.

The second host is usually some Invertebrate (Mollusc, Worm, Crustacean, Insect larva, &c.), but occasionally a Fish or Amphibian or even a vegetable. The tail is very often lost as the Cercaria bores its way into its host, but whether it is so or not, the Cercaria, after it has once reached a suitable post in its new host, assumes a quiescent condition, and surrounds itself with a many-layered capsule. The cephalic armature and tail (if still present) are then exuviated, and the generative organs gradually become apparent though very small. In other respects the organization is not much altered.

Though an encysted Cercaria may remain some months without further change, it eventually dies unless it be introduced into its permanent vertebrate host, an act which is usually effected by the host in which it is encysted being devoured. It then becomes freed from its capsule as a fully formed Trematode, in which the generative organs rapidly complete their development.

In some cases the Rediae or Sporocysts do not give rise to


tailed Cercariae, but to tailless forms. In such cases, as a rule, the encystment takes place in the host of the Redia or Sporocyst, but the tailless larvae sometimes pass through a free stage like the Cercariae. In the case of Distomum cygnoides, parasitic in the bladder of the Frog, the Cercaria passes directly into the adult host without the intervention of an intermediate host.

The life history of a typical entoparasitic Trematode is shortly as follows :

1 i ) It leaves the egg as a ciliated or non- ciliated free larva.

(2) This larva makes its way on to the gills or into the body of some Mollusc or other host, throws off its epidermis and becomes a Redia or Sporocyst.

(3) In the body cavity of the Redia or Sporocyst numerous tailed larvae, known as Cercariae, are developed by a process of internal gemmation.

(4) The Cercariae pass out of the body of their parent, and out of their host, and become for a short time free. They then pass into a second, usually invertebrate host, and encyst.

(5) If their second host is swallowed by the vertebrate host of the adult of the species, the encysted forms become free, and attain to sexual maturity.

The majority of these stages are simply parts of a complicated metamorphosis, but in the coexistence of larval budding (giving rise to Cercariae or fresh Rediae) with true sexual reproduction there is in addition a true alternation of generations.

Polystomeae. The ova of the Polystomeae are usually large and not very numerous, and they are in most cases provided with some process for attachment. Some species of Polystomeae, e.g. Gyrodactylus, are however viviparous. The young leave the egg in a nearly perfect state, and at the utmost undergo a slight metamorphosis and no alternations of generations. Some however (Polystomum, Diplozoon) are provided with temporary cilia, but the number investigated is too small to determine whether ciliation is the rule or the exception. The ciliated larvae have a short free existence. The cilia are developed on special cells which may be arranged in transverse bands in the same way as in the larvae of many Chaetopods, but are not, in the larvae at present known, distributed uniformly. When the free larvae become parasitic the cells with cilia shrink up.

B. II. H

2IO ' 1'^TODA.

In Polystotninn inlc^rrimum, which lives in the urinary bladder of Rana temporaria, the eggs when laid in the spring pass out into the water. The segmentation is complete, and the embryo when hatched is provided with most of the adult organs, but presents certain striking larval characters. It has five rings of ciliated cells. Three of these are placed anteriorly, and are especially developed on the ventral surface, the posterior one being incomplete dorsally ; two are placed posteriorly, and are especially developed on the dorsal surface. Anteriorly there is a tuft of cilia.

The larva itself resembles somewhat an adult Gyrodactylus, and is provided (i) with a large posterior disc armed with hooks, and (2) with two pairs of eyes which persist in the adult state. After a certain period of free existence the larva attaches itself to the gills of a tadpole. The rings of ciliated cells shrink up, and some of the six pairs of suckers found in the adult commence to be formed on the posterior disc. When the bladder of the tadpole is developed, the young Polystomum passes down the alimentary tract to the cloaca, and thence to the urinary bladder, where it slowly attains to sexual maturity. When the larva becomes attached to the gills of a very young tadpole, its development is somewhat more rapid in consequence of better nutrition from the more delicate gills. It then reaches its full development in the gill cavity, and. though smaller and provided with differently organised generative organs to the normal form, produces generative products and dies without being transported to the bladder (vide Zeller, Nos. 216 and 217).

The ova of Diplozoon, a form parasitic on the gills of freshwater fish (Phoxinus, etc.), are provided with a long spiral filament (Zeller, No. 215). The embryo has five ciliated areas, four lateral and one posterior. The young form is known as Diporpa. Sexual maturity is not attained till two individuals unite permanently together. They unite by the ventral sucker of each of them becoming attached to the dorsal papilla of the other. Subsequently these parts coalesce, and the ventral suckers disappear in the process. Gyrodactylus, parasitic, like Diplozoon, on the gills of freshwater fishes (Gasterosteus, etc.), is remarkable for its mode of reproduction. It is viviparous, producing a single young one at a time, and, what is still more remarkable, the young while still within its parent produces a young one, and this again a young one, so that three generations may be present within the parent. It seems probable that the second and third generations are produced asexually, the generative organs not being developed ; while the young Gyrodactylus of the first generation springs from a fertilized ovum (Wagener, No. 214).


On anatomical grounds the affinity of the Cestoda to the Trematoda has been insisted on by the majority of anatomists. The existence of such intermediate forms as Amphilina tends to


strengthen this view ; and the striking resemblances between the two groups in the structure of the egg and characters of the metamorphosis appear to me to remove all doubt about the matter.

The ripe egg is formed of a minute germ enveloped in yolk cells, the whole being surrounded by a membrane, which is very delicate in most forms, but in certain types has a firmer consistency, and is provided with an aperture, covered by an operculum, by which the larva escapes.

The early development, up to the formation of a six-hooked larva, generally takes place in the uterus, but in the types with a firmer egg-shell it takes place after the egg has been deposited in water.

The segmentation (E. van Beneden, No. 218, Metschnikofif, No. 228) is complete, and during its occurrence the yolk cells surrounding the germ are gradually absorbed, so that the mass of segmentation spheres grows in size, till at the close of segmentation it fills up nearly the whole egg-shell.

As was first shewn by Kolliker for Bothriocephalus salmonis, the embryonic cells separate themselves at the close of segmentation into a superficial layer and a central mass.

The further development takes place on two types. In the cases where the egg-shell is strong, and the egg is laid prior to the formation of the embryo, a ciliated larva is developed (Bothriocephalus latus, ditremus, Schistocephalus dimorphus, Ligula simplicissima, etc. 1 ).

Of these forms Bothriocephalus latus may be taken as type.

The development of the embryo requires many months for its completion. The outer layer becomes ciliated while the central mass has already become developed into a six-hooked embryo. The embryo leaves its shell by the opercular aperture, and for some time swims rapidly about by means of its long cilia. The ciliated coating is eventually stripped off, and the six-hooked larva emerges.

In the second type of embryo the external cellular layer does not become ciliated. This is the most usual arrangement, and is even found in many species of Bothriocephalus.

1 Vide for list of such forms at present known Willemoes Suhm, No. 231.



The central mass of cells becomes developed, as in the other type, into a six-hooked (rarely four-hooked) embryo (fig. 96 G), but the superficial layer separates from the central, and either disappears or becomes (Bothriocephalus proboscideus] a cuticular layer. Between the six-hooked embryo and the outer layer of cells one or more thick membranes become deposited (E. van Beneden). The eggs are carried out of the alimentary canal in the proglottis and transported to various situations on land or in water. They usually remain within the proglottis, invested by their thick shell, till taken up into the alimentary canal of a suitable host, or they may be swallowed after the death and decay of the proglottis. They are subsequently hatched after their shell has become softened by the action of the digestive fluids.

Before proceeding to describe their further history, the close resemblance between the first developmental stages of Cestoda, especially in the case of the ciliated larvae, and those of Trematoda, may be pointed out.

In both there is a ciliated larva, and in both there is developed


A. Cysticercus. H. and C. Cysticerci in the everted (B) and inverted (C) condition. I), ("'minis. K. and F. I )iat;rams of Kchinococcus. It is most probable that T;unia heads are not developed directly from the wall of the cyst as represented in the diagram, (i. Six-hooked embryo.

within the ciliated skin a second larva, which becomes freed by the stripping off of the ciliated skin.

The type of development has moreover many analogies with that of the Nemcrtine larva of Desor, p. 163 (cf. Mctschnikoff), and is probably like that an abbreviated record of a long history.

The suitable host for the six-hooked embryo to enter is


rarely the same as the host for the sexual form. The embryos having become transported into the alimentary canal of such a host, and become free, if previously invested by the egg-shell, soon make their way, apparently by the help of their hooks, through the wall of the alimentary tract, and are transported in the blood or otherwise into some suitable place for them to undergo their next transformation. This place may be the liver, lungs, muscles, connective tissue, or even the brain (e.g. Ccenurus cerebralis in the brain of sheep).

Here they become enclosed in a granular deposit from the surrounding tissues, which becomes in its turn enclosed in a connective-tissue coat. Within lies the solid embryo, the hooks of which in many cases disappear or become impossible to make out. In other forms, e.g. Cysticercus limacis, they remain visible, and then mark the anterior pole of the worm (fig. 98, c.}. The central part of the body next becomes transformed into a material composed of clear non-nucleated vesicles. Accompanying these changes the embryo grows rapidly in size ; a cuticle is deposited by its outer layer, in which also an external layer of circular muscular fibres and an internal layer of longitudinal fibres become differentiated, and internal to both there is formed a layer of granular cells.

With the rapid growth of the body a central cavity is formed, which becomes filled with fluid, and the embryo assumes the form of a vesicle. At the same time a system of excretory vessels, sometimes opening by a posterior pore, becomes visible in the wall of the vesicle.

The embryo has now reached a condition in which it is known as a cystic- or bladder-worm, and may be compared in almost every respect with the sporocyst of a Trematode (Huxley).

The next important change consists in the development of a head, which becomes the head of the adult Tamia. This is formed in an involution of the outer wall of the anterior extremity of the cystic worm. This involution forms a papilliform projection on the inner surface of the wall of the cystic worm, with an axial cavity opening by a pore on the outer surface. The layer of cells forming the papilla soon becomes divided into two laminae, of which the outer forms a kind of investing membrane for the papilla. The papilla itself now becomes

2I 4


Kic. 97. CYS TICERCUS CELLULOSE. (From Gefenbaur, after von iebold.)

a. Caudal vesicle, c. Anterior part of body, d, head.

moulded into a Cestode head, which however is developed in an inverted position. The suckers and hooks (when present) of the head are developed on a surface bounding the axial lumen of the papilla, which is the true morphological outer surface, while the apparent outer surface of the papilla is that which eventually forms the interior of the (at first) hollow head. Before the external armature of the head has become established, four longitudinal excretory vessels, continuous with those in the body of the cystic worm, make their appearance. They are united by a circular vessel at the apex of the head. The development is by no means completed with the simple growth of the head, but the whole inverted papilla continues to grow in length, and gives rise to what afterwards becomes part of the trunk. The whole papilla eventually becomes everted, and then the cystic worm takes the form (fig. 97) of a head and unsegmented trunk with a vesicle the body of the cystic worm attached behind. The whole larva is known as a Cysticercus. The term scolex, which is also sometimes employed, may be conveniently retained for the head and trunk only. The head differs mainly from that of the adult in being hollow.

There are great variations in the relative size of the head and the vesicle of Cysticerci. In some forms the vesicle is very small (fig. 98), e.g. Cysticercus limacis ; it is medium-sized in Cysticercus cellulosce (fig. 97), and in some forms is much larger. The embryonic hooks, when they persist, are found at the junction of the trunk and the vesicle (fig. 98 A, c}. Though the majority of cystic worms only develope one head, this is not invariably the case. There is a cystic worm found in the brain of the sheep known as Ccenurus cerebralis the larva of Tcenia caenurus, parasitic in the intestine of the dog which forms an exception to this rule. There appears, to start with, a tuft of three or four heads, and finally many hundred heads are developed (fig. 96 D). They



A. Head involuted. B. Head everted.

a. Scolex. />. caudal vesicle. c. (in A) six embryonic hooks.

are arranged in groups at one (the anterior?) pole of the cystic worm.


A still more complicated form of cystic worm is that known as Echinococcus, parasitic in the liver, lungs, etc. of man and various domestic Ungulata. In the adult state it is known as Tcenia echinococcus and infests the intestine of the dog. The cystic worm developed from the six-hooked embryo has usually a spherical form, and is invested in a very thick cuticle (fig. 96 E and F, and fig. 99). It does not itself directly give rise to Taenia heads, but after it reaches a certain size there are formed on the inner side of its walls small protuberances, which soon grow out into vesicles connected with the walls of the cyst by narrow stalks (figs. 96 F and 99 C). In the interior of these vesicles a cuticle is developed. It is in these secondary vesicles that the heads originate. According to Leuckart, they either arise as outgrowths of the wall of the vesicle on the inner face of which the armature is developed, which subsequently become involuted and remain attached to the wall of the vesicle by a narrow stalk, or they arise from the first as papilliform projections into the lumen of the vesicle, on the outer side of which the armature is formed. Recent observers only admit the second of these modes of development. The Echinococcus larva, in addition to giving rise to the above head-producing vesicles, also gives rise by budding to fresh cysts, which resemble in all respects the parent cyst. These cysts may either be detached in the interior (fig. 96 F) of the parent or externally. They appear to spring in most cases from the walls of the parent cyst, but there are some discrepancies between the various accounts of the process. In the cysts of the second generation vesicles are produced in which new heads are formed. As the primitive cyst grows, it naturally becomes more and more complicated, and the number of heads to which one larva may give rise becomes in this way almost unlimited.

Cysticerci may remain a long time without further development, and human beings have been known to be infested with an Echinococcus cyst for over thirty years. When however the Cysticercus with its head is fully developed, it is in a condition to be carried into its final host. This takes place by the part of one animal infested with cysticerci becoming eaten by the host in question. In the alimentary canal of the final host the connective-tissue capsule is digested, and then the vesicular caudal appendage undergoes the same fate, while the head, with its suckers and hooks, attaches itself to the walls of the intestine. The head and rudimentary trunk, which have been up to this time hollow, now become solid by the deposition of an axial tissue; and the trunk very soon becomes divided into segments, known as proglottides (fig. 99 A). These segments are not formed in the same succession as those of Chaetopods ; the



youngest of them is that nearest to the head, and the oldest that furthest removed from it. Each segment appears in fact to be a sexual individual, and is capable of becoming detached and leading for some time an independent existence. In some cases, e.g. Cysticcrcus fasciolaris, the segmentation of the trunk may take place while the larva is still in its intermediate host.

The stages in the evolution of the Cestoda are shortly as follows :

1. Stage with embryonic epidermis either ciliated (Bothriocephalus, etc.) or still enclosed in the egg-shell. This stage corresponds to the ciliated larval stage of the Trematoda.

2. Six-hooked embryonic stage after the embryonic epidermis has been thrown off. During this stage the embryo is transported into the alimentary tract of its intermediate host, and boring its way into the tissues, becomes encapsuled.

3. It develops during the encapsuled state into a cystic worm, equivalent to the sporocyst of Trematoda.

4. The cystic worm while still encapsuled develops a head with suckers and hooks, becoming a Cysticercus. In some forms (Ccenurus, Echinococcus) reproduction by budding takes place at this stage. The head and trunk are known as the scolex.

5. The Cysticercus is transported into the second and permanent host by the infested tissue being eaten. The bladderlike remains of the cystic worm are then digested, and by a process of successive budding a chain of sexual proglottides are formed from the head, which remains asexual.

The above development " is to be regarded as a case of


A. Tsenia head or scolex. a. hooks, b. suckers, c. cilia in water vessel, d. refracting particles in body wall.

B. single hooks.

C. portion of cyst. a. cuticle. b. membranous wall of primary cyst. c. and e. scolex heads, d. secondary cyst.



FIG. 99 A. TETRARHYNCUS. (From Gegenbaur ; after Van Beneden.)

A. Asexual state.

B. Sexual stage with ripe proglottides.

complicated metamorphosis secondarily produced by the necessities of a parasitic condition, to which an alternation of sexual and gemmiparous generations has been added. The alter- A nation of generations only occurs at the last stage of the development, when the so- B| called head, without generative organs, produces by budding a chain of sexual forms, the embryos of which, after passing through a complicated metamorphosis, again become Cestode heads.

In the case of Ccenurus and Echinococcus two or more asexual generations are interpolated between the sexual ones. It is not quite clear whether the production of the Taenia head from the cystic worm may not be regarded as a case of budding. There are some grounds for comparing the scolex to the Cercaria of Trematodes, cf. Archigetes.

As might be anticipated from the character of the Cestode metamorphosis, the two hosts required for the development are usually forms so related that the final host feeds upon the intermediate host. As familiar examples of this may be cited the pig, the muscles of which may be infested by Cysticercus cellulosce, which becomes the Tcenia solium of man. Similarly a Cysticercus infesting the muscles of the ox becomes the TcBnia mediocanellata of man. The Cysticercus piscifonnis of the rabbit becomes the Tcenia serrata of the dog. The Coenurus cerebralis of the sheep's brain becomes the Tcenia ccenurus of the dog. The Echinococcus of man and the domestic herbivores becomes the Tcenia echinococcus of the dog.

Cystic worms infest not only Mammalian forms, but lower Vertebrates, various fishes which form the food of other fishes, and Invertebrates liable to be preyed on by vertebrate hosts. So far the Cestodes (except Archigetes) are only known to attain sexual maturity in the alimentary tracts of Vertebrata.

The rule that the intermediate host is not the same as the final host does not appear to be without exception. Redon 1 has shewn by experiments on himself that a Cysticercus (celluloses) taken from a human subject developes into Tcenia solium in the intestines of a man. Redon took four cysts of a Cysticercus from a human subject, and after three months passed some proglottides, and subsequently the head of Tania solium.

1 Annal. d. Scien. Nat., 6th Series, Vol. vi. 1877.


Some important variations of the typical development are known.

The so-called head or scolex may be formed without the intervention of a cystic stage. In Archigetes (Leuckart, No. 227), which infests, in the Cysticercus condition, the body-cavity of various invertebrate forms (Tubifex, etc.), the six-hooked embryo becomes elongated and divided into two sections, one forming the head, while the other, with the six embryonic hooks, forms an appendage, homologous with the caudal vesicle of other Cysticerci.

The embryo of Tcenia elliptica similarly gives rise to a Cysticercus infesting the dog-louse (Trichodectes cants], without passing through a vesicular condition ; but the caudal vesicle disappears, so that it forms simply a scolex. These cases may, it appears to me, be probably regarded as more primitive than the ordinary ones, where the cystic condition has become exaggerated as an effect of a parasitic life.

In some cases the larva of a Taenia has a free existence in the scolex condition. Such a form, the larva of Phyllobothrium, has been observed by Claparede 1 . It was not ciliated, and was without a caudal vesicle; and was no doubt actively migrating from an intermediate host to its permanent host.

Scolex forms, without a caudal vesicle, are found in the mantle cavity of Cephalopoda, and appear to be occupying an intermediate host in their passage from the host of the cystic worm to that of the sexual form.

Archigetes, already mentioned, has been shewn by Leuckart (No. 227) to become sexually mature in the Cysticercus state, and thus affords an interesting example of paedogenesis. It is not known for certain whether under normal circumstances it reaches the mature state in another host.

Amphilina. The early stages of this interesting form have been investigated by Salensky (No. 229), and exhibit clear affinities to those of the true Cestoda. An embryonic provisional skin is formed as in Cestodes ; and pole-cells also appear. Within the provisional skin is formed an embryo with ten hooks. After hatching the provisional skin is at once thrown off, and the larva, which is then covered by a layer of very fine cilia, becomes free. The further metamorphosis is not known.

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