The Works of Francis Balfour 2-10
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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
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.
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Vol II. A Treatise on Comparative Embryology (1885)
Chapter X. Polyzoa
THE development of the larvae of Pedicellina is known from the researches of Hatschek (No. 299) far more completely than that of Loxosoma, though it does not apparently differ from it except in certain details. In both the known Entoproctous genera the segmentation is regular or nearly so, though Hatschek believes that he has detected in Pedicellina a slight difference between the two first segmentation spheres, and regards them as constituting the animal and vegetative poles of the embryo. The segmentation in Pedicellina, to which genus alone the remainder of the description applies, results in the formation of a singlelayered blastosphere, with a small segmentation cavity, in which the animal and vegetative poles can readily be distinguished owing to the smaller size of the cells at the animal pole.
The hypoblast cells and the vegetative 2 pole become invagi
1 The classification of the Polyzoa adopted in this chapter is shewn in the subjoined table :
I. Entoprocta. II. Ectoprocta.
1. GYMNOUEMATA \b. Ctenostomata.
3. PODOSTOMATA (Rhabdopleuro).
2 The succeeding statements about the gastrula are derived from Hatschek. According to Salensky a segmentation cavity is not present, and the hypoblast would seem to be formed by delamination or epibole. Barrois finds a gastrula in both Loxosoma and Pedicellina, but gives no details. Uljanin finds a segmentation cavity in Pedicellina, and Schmidt would appear to have observed a gastrula stage in Loxosoma. None of the accounts we have can be compared in fulness of detail to that of Hatschek.
. POLYZOA. 293
nated in the normal manner (fig. 128 A), the blastopore becomes narrowed to a slit with an antero-posterior direction, i.e. parallel to the line connecting the mouth and anus in the adult. At the hinder extremity of the blastopore there are present two conspicuously large cells (fig. 128 B, me), one on each side of the middle line. These cells give rise to the fnesoblast. On the completion of the invagination the mesoblasts become covered by the epiblast (fig. 128 C, me). The blastopore then closes, but in the position it occupied the epiblast becomes thickened to form the rudiment of the vestibule, which at this stage constitutes a disc marked off by a shallow groove from the remainder of the body.
FIG. 128. THREE STAGES IN THE DEVELOPMENT OF PEDICELLINA ECHINATA. (After Hatschek.)
s.c. segmentation cavity ; a.e. archenteron ; ep. epiblast ; me. mesoblast ; hy. hypoblast.
A is the commencing gastrula stage from the side in optical section.
B is a slightly later stage from above in optical section. It shews the two primitive mesoblast cells.
C is a later stage after the closure of the blastopore, viewed from the side in optical section.
At the anterior extremity of this disc an invagination arises to form the oesophagus (fig. 129 A, oe) ; and not long afterwards a posterior invagination to form the rectum (fig. 129 B, an.i). The oral disc and the oesophagus are richly ciliated. The cesophagus first, and afterwards the rectum unite with the archenteron (fig. 130), the walls of which soon become differentiated into a stomach and intestine, and on the upper wall of the former the hepatic cells become especially conspicuous (fig. 130).
During the completion of the alimentary canal a number of important structures is formed. The disc in which the oral and anal apertures are situated becomes converted into a true vesti
FlG. 129. TWO STAGES IN THE DEVELOPMENT OF PEDICELLINA. (After
oe. oesophagus ; ae. archenteron ; an.i. anal invagination ; f. fold of epiblast ; f.g. ciliated disc ; x. problematical body derived from hypoblast (probably a bud).
bule. On its floor, between the mouth and the anus, there arises a marked prominence with a tuft of cilia (fig. 130 B), which persists in the adult.
This prominence is perhaps equivalent to the epistome of the Phylactolsemata and the disc-like organ of Rhabdopleura, which Lankester has compared to the molluscan foot 1 .
Very shortly after the first formation of the vestibule there appears at the opposite end of the larva a thickening of the epiblast, which soon becomes invaginated, and forms an eversible pit (fig. 129 A and B, /.-.). Round its mouth there is formed a ring of stiff cilia (fig. 130, f.g.). This organ is very possibly equivalent to the cement gland described by Kowalevsky in the adult Loxosoma. I shall speak of it as the ciliated disc.
The epiblast cells early secrete a cuticle.
The two mesoblast cells soon increase by division, and occupy the space between the alimentary canal and the body wall. They do not become divided into a splanchnic and somatic layer ; but give rise to the interstitial connective tissue
1 Lankester. "Remarks on the Affinities of Rhabdopleura." Quart. J. of Micro. Science, Vol. XIV. 1874.
and muscles. From the mesoblast there is also formed, according to Hatschek, a pair of ciliated excretory canals, in the space between the mouth and anus (fig. 130 B, npk.). The development of the nervous system has not been observed.
At a comparatively late stage in the development there is formed round the edge of the vestibule a ring of long cilia (fig. 1306, m.).
A remarkable organ makes its appearance on the dorsal side of the oesophagus (the side opposite the adult ganglion) formed of an oval mass of cells attached to the epiblast at the apex of a small ciliated papilla (fig. 130 A and B, *.). This organ will be
FlG. 130. TWO STAGES IN THE DEVELOPMENT OF PEDICELLINA. (After
v. vestibule ; m. mouth ; /. liver ; kg. hind-gut ; a. anus ; and. anal invagination ; nph. duct of kidney \ fg. ciliated disc ; x. dorsal organ (probably bud).
spoken of as the dorsal organ. According to Hatschek it develops as a solid outgrowth of the hypoblastic walls of the mesenteron shortly before the mesenteron joins the oesophagus (fig. 129 B, jr.). The cells composing it arrange themselves as a sack, which acquires an external opening on the dorsal surface (fig. 130 A, jr.). In a later stage the lumen of the sack disappears, but at the junction of the organ with the epiblast a pit
is formed, lined with ciliated cells, which is capable of being protruded as a papilla. The organ itself becomes invested by a lining of cells, which Hatschek regards as mesoblastic. A nearly similar organ to this is found in the embryo of Loxosoma [Vogt (No. 302) and Barrois (No. 298)]. Here however it is double, and forms a kind of disc connected with two eye spots.
Hatschek has made with reference to the dorsal organ the extremely plausible suggestion that it is a rudimentary bud, and that the hypoblastic sack it contains gives rise to the hypoblast of the young polype developed from the bud. Although, owing to the deficiency of our observations on the attachment of the larva, this suggestion has not received direct confirmation, yet the relations of dorsal organs in Pedicellina and Loxosoma respectively strongly confirm Hatschek's view of their nature. Both of these forms increase in the adult state by budding : in Pedicellina there is a single row of buds formed successively on the dorsal side of the stem, corresponding with the single dorsal organ of the embryo ; while in Loxosoma a double row of buds, right and left, is formed, in correspondence with the double nature of the dorsal organ.
As to the mode of attachment of the embryo next to nothing is known, the few observations we have being due to Barrois. From these observations it would appear probable that the larva, as is usual amongst Polyzoa, does not become directly converted into the permanent form, but that, on becoming fixed, it undergoes a metamorphosis in the course of which its organs atrophy. I would venture to suggest that the whole free-swimming larva atrophies, while the dorsal organ alone develops into the fixed form 1 .
Although the changes which take place during budding do not fall within the province of this work, it may be well to state that Hatschek has observed during this process the development of the nervous system and the generative organs. The nervous system arises as an unpaired thickening of the epiblastic floor of the vestibule, between the mouth and the anus. On becoming constricted off from the epiblast the nerve ganglion contains a central cavity which afterwards vanishes.
The generative organs originate as a pair of specially large mesoblast cells in the space between the stomach and the floor of the vestibule. These two cells, surrounded by an investment of flattened mesoblast cells, sub 1 My view of the metamorphosis which takes place during the fixation of the larva involves the supposition that in Loxosoma, about the attachment of which we know absolutely nothing, two buds are directly formed in accordance with the double nature of the dorsal organ.
sequently divide and form two masses. At a still later period each mass divides into an anterior and a posterior part ; the former giving rise to the ovary, the latter to the testis. The similarity of this mode of development of the generative organs to that observed by Butschli in Sagitta, which is described in the sequel, is very striking.
Although the embryology of the Ectoprocta has been investigated by a very considerable number of the distinguished naturalists of the century, many points connected with it still stand in great need of further elucidation. The original nature of the embryo was rightly interpreted by Grant, Dalyell and other naturalists, but it was not till Huxley demonstrated the presence of both the ovary and testis that the true sexual origin of the embryo in the ovicells became an established fact in science. The recent memoir of Barrois (No. 298), though it contains the record of a vast amount of research, and marks a great advance in our knowledge, still leaves a great number of points, both with reference to the early development and to the larval metamorphosis in a very unsatisfactory condition.
Four larval forms can be distinguished, viz.
(1) A larval form which with slight modifications is common to all the genera of the Chilostomata (except Membranipora and Flustrella) and of the Ctenostomata.
(2) A bivalved larva of Membranipora known as Cyphonautes y the true nature of which was first recognized by Schneider (No. 322), and the closely allied larva of Flustrella.
(3) The typical Cyclostomatous larva, for the first full description of which we are indebted to Barrois (No. 298).
(4) The larva of the Phylactolaemata.
Chilostomata and Ctenostomata. As an example of the first type of larvae, Alcyonidium my till, one of the Ctenostomata, may be conveniently selected for description, as having been more completely worked out by Barrois than perhaps any other form. The segmentation commences in the normal manner by the appearance of two vertical furrows followed by an equatorial furrow, which divide the ovum into eight equal spheres. The stage with eight spheres is followed, according to Barrois, by one with sixteen, formed in a remarkable manner by the simultaneous
appearance of two vertical furrows, both parallel to one of the original vertical furrows, so that the segmentation spheres at this stage are arranged in two layers of eight each. In the next stage segmentation takes place along two fresh vertical planes, similar to those of the last stage, but at right angles to them, and therefore parallel to the second of the two primitive vertical furrows. At the close of this stage there are thirty-two cells arranged in two layers of sixteen eachj and when viewed from the surface each of these layers presents a regularly symmetrical pattern. Up to the stage with sixteen cells the two poles of the egg, separated by the primitive equatorial plane of segmentation, remain equal, but during the stage with thirty-two cells a peculiar change takes place in the character of the cells at the two poles. At the one pole, which will be spoken of as the oral pole, the four central cells become much larger than the twelve peripheral cells.
The stages immediately following are still involved in much obscurity, and have been described very differently by Barrois in his original memoir (No. 298), and in a subsequent note (No. 307) 1 . In the latter he states that the four large cells of the oral face become enclosed by the division and growth of the twelve peripheral cells. They are thus carried into the interior of the ovum ; and there divide into a central vitelline mass the hypoblast and a peripheral mesoblastic layer.
The eight peripheral cells of the aboral pole divide vertically, and, owing to the eight central cells at the aboral pole dividing transversely so as to form a protuberance on the aboral surface, they constitute a transverse ring of large cells round the ovum, which become ciliated and constitute the main ciliated band of the embryo, corresponding to the ciliated band at the edge of the vestibule of the entoproctous larvae. They divide the embryo into an aboral and an oral region. The central part of the aboral projection forms a structure which I shall speak of as the ciliated disc. It probably corresponds with the ciliated disc in the Entoprocta. An invagination is next formed on the oral
1 The note (No. 307) refers in the first instance to the changes in the larvae of the Chilostomata, but the similarity of the larvoe of the Ctenostomata to those of the Chilostomata renders it practically certain that the corrections, in so far as they apply to the one group, apply also to the other.
surface, which gives rise to a sack opening to the exterior (fig. 131, st.). This was originally held by Barrois to be the stomach ; but Barrois now prefers to call it ' the internal sack.' To my mind it is probably the stomodaeum. The embryo has become in the meantime laterally compressed, and, at what I shall call the anterior end of the oral disc, a structure makes its appearance (fig. 131, m), which is probably homologous with the dorsal organ of the larva of Pedicellina and may go by the same name. It was originally interpreted by Barrois as the pharynx 1 .
The larva, having now acquired all the important structures it is destined to possess, becomes free. It is shewn in fig. 131 ; the oral face being turned upwards. There are two rings of
FIG. 131. FREE-SWIMMING LARVA OF ALCYONIDIUM MYTILI. (After Barrois.) m (?) dorsal organ ; st. stomodseum (?) ; s. ciliated disc.
cilia, one round the edge of the ciliated disc, and a second with larger cilia on the ring of large cells described above. This ring projects somewhat ; its projecting edge being directed towards the ciliated disc. The dorsal organ (m?) is placed on the oral face at the bottom of an elongated groove, in front of which is a bunch of long cilia or flagella. Two long flagella are also developed at the posterior extremity of the oral face, and two pairs (an anterior and a posterior) of eye-spots also appear. Towards the posterior extremity of the oral face is seen a body marked st, which forms the internal sack. If I am
1 The interpretation of the larvae given in the text must be regarded as somewhat tentative. The opacity of the free larvae is very great, and almost every one of the numerous authors who have worked on these larvae have arrived at different conclusions, as to the physiological significance of the various parts.
right in regarding this as the stomodaeum, it is probable that it never unites with the invaginated hypoblast, and that the alimentary tract of the larva remains therefore permanently in an imperfect condition.
Careful observations have been made by Repiachoff (No. 318) on the early development of Tendra, which accord in some respects with the results arrived at by Barrois in his second memoir. The observations are not, unfortunately, carried down to the complete development of the larva.
The ovum divides in the normal way into two and then four uniform segments. These four next become divided by an equatorial furrow into four dorsal and four ventral segments, the former constituting the aboral pole and forming the epiblast, and the latter the oral pole. The stages with sixteen and thirty-two cells appear to be formed in the same manner as in Alcyonidium but between the two layers of cells forming the oral and aboral poles a well-marked segmentation cavity arises at the stage with sixteen segments. At the stage with thirty-two cells the four middle cells of the oral side, which are larger than the others, become divided into two tiers, in such a manner as to form a prominence projecting into the segmentation cavity. By the appearance of a lumen in this prominence it becomes converted into an archenteron, which communicates with the exterior by a blastopore in the middle of the oral surface. The blastopore becomes eventually closed.
The archenteric sack of Repiachoff is clearly the same structure as Barrois' four invaginated cells of the oral face, their further history has unfortunately not been followed out by Repiachoff.
The free larva swims about for some time, and then fixes itself and undergoes a metamorphosis; but the exact course of this metamorphosis is still very imperfectly known.
According to the latest statements of Barrois the attachment takes place by the oral face 1 . The ciliated disc, which in the free larva forms a kind of cup directed towards the aboral end, turns in upon itself towards the oral face. It subsequently undergoes degeneration and forms a nutritive or yolk-mass. The skin of the larva after these changes gives rise to the ectocyst or cell of the future polype. The future polype itself appears to originate, in part at any rate, from the so-called dorsal organ*.
1 Barrois himself held the opposite view in his earlier memoir, and other observers have done the same.
a The statements on this head are so unsatisfactory and contradictory that it does not appear to me worth while quoting them here ; even the latest accounts of Barrois, which entirely contradict his early statements, can hardly be regarded as satisfactory.
The first distinct rudiment of the polype appears as a white body, which gradually develops into the alimentary canal and lophophore. While this is developing the ectocyst grows rapidly larger, and the yolk in its interior separates from the walls and occupies a position in the body cavity of the future polype, usually behind the developing alimentary canal. According to Nitsche (No. 316) it is attached to a protoplasmic cord (funiculus) which connects the fundus of the stomach with the wall of the cell. It is probably (Nitsche, etc.) simply employed as nutritive material, but, according to Barrois, becomes converted into the muscles, especially the retractor muscles.
Adopting the hypothesis already suggested in the case of the Entoprocta the metamorphosis just described would seem to be a case of budding accompanied by the destruction of the original larva.
This view of the nature of the post-embryonic metamorphosis is apparently that of Claparede and Salensky, and is supported by Claparede's statement that the formation of the first polype ' resembles to a hair ' that of the subsequent buds. The mode of budding would, however, appear to present certain peculiarities, in that the whole larval skin passes directly into the bud, while from the rudimentary bud of the larva the lophophore and alimentary tract only of the fixed polype are formed.
Flustrella and Cyphonautes. The next group of larval forms is that of which Cyphonautes is the best known type. The larvae composing it at first sight appear to have but little in common with the larvae hitherto described. The researches of Barrois (No. 298) and Metschnikoff (No. 314), (but especially those of the former on the early stages of Flustrella hispida, the larva of which is very similar in form to Cyphonautes, though without so great a complexity of organisation), have given a satisfactory basis for a general comparison of Cyphonautes with other ectoproctous larvae.
The segmentation and early stages of the embryo of Flustrella resemble closely those of Alcyonidium. A projecting ring of large cells is formed, dividing the larva into oral and aboral parts. The oral part soon however becomes very small as compared with the aboral, and becomes vertically flattened so as to be nearly on a level with the ring of large cells. In the next stage the flattening becomes completed ; and the ring of large cells
surrounds, like the vestibule of the Entoprocta, a flat oral disc. The aboral side is dome-shaped, and forms the greater part of the embryo.
FIG. 132. ADVANCED LARVA OF FLUSTRELLA HISPIDA. (After Barrois.) m (?) groove above dorsal organ ; Ph. dorsal organ ; st. stomodoeum (?) ; s. ciliated disc at aboral end of body.
In the next stage a small disc the ciliated disc is formed in the middle of the aboral dome. The larva becomes laterally compressed. The ring of large cells which now constitute the edge of the vestibule is covered, as in the larva of Pedicellina, by cilia, which are specially long in front of the dorsal organ.
In the next stage the ciliated disc (fig. 132, s.) becomes reduced in size, but surmounted by a ring of cilia round the edge, and a tuft of cilia in the centre. The chief difference between this larva and that of Alcyonidium depends on the small size of the ciliated disc, and the oral position of the ciliated ring in the former. There are intermediate types between these forms of larvae.
This stage immediately precedes the liberation of the larva. The free larva differs from that in the ovicell mainly in the possession of a shell formed as a cuticular structure, composed of two valves placed on the two sides of the embryo. The aboral ciliated disc, still more reduced in size, loses its cilia, and becomes enclosed between the two valves of the shell.
The post-embryonic metamorphosis follows, so far as is known, the course already described for the larva of Alcyonidium.
Cyphonautes (fig. 133) forms at certain seasons of the year one of the commonest captures in the surface net. It was originally described by Ehrenberg, but the important discovery of its true nature as the larva of Membranipora (the common species C. compressus is the larva of Mem. pilosa), a genus of the chilostomatous Polyzoa, was made by Schneider (No. 322). The younger stages of the larva have not been worked out, but from a comparison with the last described larva it is easy to make out the general relationship of the parts. The larva has a triangular form with an aboral apex, corresponding with the summit of the dome of the Flustrella larva, and an oral base. It is enclosed in a bivalve shell, the two valves of which meet along the two sides, but are separate along the base. At the apex an opening is left between the two valves, through which a ciliated disc (/. g) of the same character and nature as that of previous larvae can be protruded.
The oral side or base is girthed by a somewhat sinuous ciliated edge, which is continued round the anterior and posterior extremities of the oral disc. It is no doubt equivalent to the ciliated ring of other larvae. Two openings are present on the oral face, both enclosed in a special lobe of the ciliated ring. The larger of these leads into a depression, which may be called the vestibule; and is situated on the posterior side of the oral surface. The smaller of the two, on the anterior side, leads into a cavity which is apparently (Hatschek) equivalent to the rudimentary bud or dorsal organ of other larvae. The deeper part of the vestibule leads into the mouth (m) and oesophagus ; the latter is continued till close to the
CYPHONAUTES (LARVA OF MEM( After Hatschek.)
FIG. 133 BRANIPORA).
m. mouth; a. anus; f.g. ciliated disc; x. problematical body (probably a bud).
apex of the larva, there bends upon
itself, dilates into a stomach, and is continued parallel to the oesophagus as the rectum which opens by an anus (a 1 ) at the posterior end of the vestibule. A peculiar paired organ is situated on each side nearly above the stomach. Its nature is somewhat doubtful. It was regarded as muscular by Claparede (No. 309), though this, as shewn by Schneider, is no doubt a mistake. Allman (No. 305) regards it as hepatic, and Hatschek as a thickening of the epidermis. Close to each of these organs is a small body regarded by Claparede as an accessory muscle. It is placed in the normal position for a Polyzoon ganglion, and may perhaps be therefore regarded as nervous in nature. Allman points out its similarity to a bilobed ganglion, but is not inclined to take this view of it. The constitution of the parts contained in the anterior cavity (x) is somewhat obscure. The most elaborate descriptions of them are given by Schneider and Allman. Lining the cavity is apparently a mass of spherical bodies, connected with which is a tongue-like process provided with long cilia, which can be protruded from the orifice. Internal to this is a striated body. A good figure of the whole structure is given by Schneider.
The general similarity of Cyphonautes to the other larvae is quite obvious from the above description and figure. In the presence of an anus, a vestibule, and possibly a nervous system, it clearly exhibits a far more complicated organisation than any other Polyzoon larvae except those of the Entoprocta.
The post-embryonic metamorphosis of Cyphonautes, admirably investigated by Schneider, takes place in the same manner as that of other larvae, and is accompanied by the degeneration of the larval organs, and the formation of a clear body, which gives rise to the alimentary cavity and lophophore of the fixed polype. The larval shell takes part in the formation of the ectocyst of the polype.
Cyclostomata. We owe to Barrois by far the fullest account of the development of the Cyclostomata, but how far his interpretations are to be trusted is very doubtful. The larvae differ very considerably from the normal larvae of the Chilostomata and Ctenostomata ; the difference being mainly due to the enormous development of the ciliated disc. Barrois has investigated the larvae of three genera, Phalangella, Crisia, and Diastopora,
and states that they very closely resemble each other. The ovum is extremely minute.
The segmentation, so far as it has been made out, is regular. During the segmentation growth is very rapid, and eventually there is formed a blastosphere many times larger than the original ovum. The blastosphere becomes flattened, and is converted into a gastrula by bending up into a cup-like form. The gastrula opening is stated to remain as the permanent mouth, which has a terminal and central position. A transverse ring-like thickening is formed round the larva, which probably corresponds with the ciliated ring of previous larvae ; and the body of the larva in front of this ring becomes ciliated. The aboral end of the larva becomes thickened, and grows out into an elongated prominence, which probably corresponds to the ciliated disc. The ring before mentioned becomes at the same time more prominent, and forms a cylindrical sheath for the ciliated disc. At the time when the larva becomes liberated from the maternal cell it has the form of a barrel with a slight constriction in the middle separating the oral from the aboral end. At the centre of the oral face is situated the mouth, leading into a wide stomach, while the aboral end is formed of the ciliated disc enclosed in its sheath. The whole surface is now ciliated. No structure equivalent to the dorsal organ or bud is described by Barrois, but in other respects, if the ciliated disc is really equivalent in the two forms, a general comparison on the line indicated above between this larva and the normal larvae of the Ctenostomata and Chilostomata seems quite possible. The fixation and subsequent development of the larva take place in the normal manner.
Phylactolaemata. The development of the phylactolaematous Polyzoa has been studied by Metschnikoff (No. 315), who describes the eggs as undergoing a complete segmentation within a peculiar brood-pouch developed from the walls of the body of the parent. After segmentation the cells of the embryo arrange themselves in two layers round a central cavity. The embryo then forms the well-known cyst, from which a colony is formed by a process of budding.
General considerations on the Larva of the Polyzoa.
The different forms of embryo amongst the Polyzoa are represented in figs. 130 B, 131, 132, and 133 in what I regard as identical positions, and fig. 133 A is a figure of what may be regarded as an idealized larval Polyzoon. In all the larvae there is present a ciliated ring, which separates an oral from an aboral face, and is apparently homologous throughout the series. In the adult it is probably represented by the lophophore. On the B. IT 20
oral face is situated in all cases the mouth, and in the entoproctous larvae and Cyphonautes also the anus. It thus appears that Cyphonautes, though the larva of an ectoproctous form, is itself entoproctous a fact which tends to shew that the Entoprocta are the more primitive forms. In all the larvae, except possibly those of the Cyclostomata, there is present on the anterior side of the mouth, in the Ectoprocta on the oral, and in the Entoprocta on the aboral side of the ciliated ring, an organ, to which is attached externally a plume of long cilia. This organ has been identified throughout the series in accordance with Hatschek's view as the dorsal organ or rudimentary bud ; but it is well to bear in mind that this identification is of a purely hypothetical character.
On the aboral side of the ciliated ring there is present in all the larv* an organ, which has been called the ,. m0 uth; an. anus;^. stociliated disc, which is probably homo- mach ' *- ciliated disc logous throughout the series. It perhaps remains in the adult of Loxosoma as the cement gland, but not in other forms.
The Polyzoa present a simple and almost certainly degraded organisation in the adult state ; it is therefore more than usually necessary to turn to their larvae for the elucidation of their affinities, and various plausible suggestions have been made as to the interpretation of the characters of the larvae.
Lankester 1 has suggested that the larvae are essentially similar to those of Molluscs. He compares the main ciliated ring to the velum, but has ingeniously suggested that it represents not the simple velar ring of most molluscan larvae, but a more extended longitudinal ring, of which the gills of Lamellibranchiata are supposed by him to be remnants, and to which the Echinoderm larvae with one continuous ciliated band furnish a parallel.
The foot he finds in the epistome of the Phylactolaemata, and the disc of Rhabdopleura both situated between the mouth and anus, and therefore in the situation of the molluscan
1 Lankester. "Remarks on the affinities of Rhabdopleura." Quart. J. of Mitro. Science, Vol. XIV. 1874.
foot. The peculiar prominence between the mouth and the anus in Pedicellina (vide fig. 130 B) and Loxosoma is probably the same structure.
Finally he identifies my ciliated disc, which as mentioned above is perhaps equivalent to the cement gland in the adult Loxosoma, as the molluscan shell-gland. Lankester's interpretations are very plausible, but at the same time they appear to me to involve considerable difficulties.
There is absolutely no evidence amongst the Mollusca of the existence of a primitive longitudinal ciliated ring, such as he supposes to have existed, and Lankester is debarred from regarding the ciliated ring of the Polyzoa as equivalent to the simple velar ring of the Mollusca, because his shell-gland lies in the centre and not as it should do on the posterior side of the ciliated ring.
Another difficulty which I find is the invariable ciliation of Lankester's shell-gland a ciliation which never occurs amongst Mollusca.
It appears to me that a more satisfactory comparison of the larvae of the Polyzoa with those of the Mollusca is obtained by dropping the view that the ciliated disc is the shell-gland, and by regarding the ciliated ring as equivalent to the velum. This mode of comparison has been adopted by Hatschek.
The larva ceases however on this view to have any special molluscan characters (except possibly the organ which Lankester has identified as the foot), and only resembles a molluscan larva to the same extent as it does a larva of the Polychaeta. The ciliated disc lies according to this view in the centre of the velar area or prae-oral lobe, and therefore in the situation in which a tuft of cilia is often present in lamellibranchiate and other molluscan larvae, and also in the larvae of most Chaetopoda. It is moreover at this point that the supra-cesophageal ganglion is always formed in the Mollusca and Chaetopoda as a thickening of the epiblast (fig. 134, .$-.), so that the thickening of the epiblast in the ciliated disc of the Polyzoa may perhaps be a rudiment of the supra-cesophageal ganglion, which entirely atrophies in the adult after the attachment has been effected in the region of this disc.
The comparison between the Polyzoon larva and that of a
Chaetopod becomes very much strengthened by taking as types Mitraria 1 (fig. 134) and Cyphonautes (fig. 133). The similarity
FIG. 134. Two STAGES IN THE DEVELOPMENT OF MITRARIA. (After Metschnikoff.)
m. mouth; an. anus; sg. supra-oesophageal ganglion; br. and b. provisional bristles; pr.b. prae-oral ciliated band.
between these two forms is so striking that I am certainly inclined to view the larvae of the Polyzoa as trochospheres similar to those of Chaetopods, Rotifera, etc., which 'become fixed in the adult by the extremity of their prce-oral lobe.
The attachment of the larva by the prae-oral lobe is not more extraordinary than the attachment of a Barnacle by its head, and after such a mode of attachment the atrophy of the supracesophageal ganglion would be only natural.
There is one important fact which deserves to be noted in the development of the Polyzoa, viz. that if the suggestion in the text as to the mode of development of the adult from the socalled larva is accepted, the Polyzoa exhibit universally the phenomenon of alternations of generations. The ovum gives rise to a free form which never becomes sexual, but produces by budding the sexual attached form.
1 The larva of Mitraria is figured with the aboral surface turned upwards, instead of downwards, as in the figure of Cyphonautes. The ciliated band is also diagrammatical ly put in black for greater distinctness.
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