[134]The following classification of the Chætopoda is adopted in the present section:I.Achæta.(Polygordius).II.Polychæta.Sedentaria.Errantia.III.Oligochæta.[135]The observations of Stossich are not thoroughly satisfactory.[136]According to Willemoes-Suhm, Terebellides strœmii is also characterised by a regular segmentation.[137]These two rings are at first (Hatschek) not quite closed dorsally, calling to mind the early condition of the Echinoderm larvæ with a præ-oral and post-oral ciliated area.[138]The structure of the ventral cord in the adult requires further elucidation.[139]For Semper’s view as to the intercalation of segments in the cephalic region,videnote onp.333.[140]It has been insisted by Semper (No.355) that certain of the anterior segments, belonging to what he regards as the head region in opposition to the trunk, become interpolated between the trunk and the head. The general evidence, founded on observations of budding, which he brings forward, cannot be discussed here. But the special instance which he cites (founded on Milne-Edwards’s (No.347) observations) of the interpolation of the head segments, bearing the gills, in Terebella appears to me quite unjustified from Milne-Edwards’s own statements; and is clearly shewn to be unfounded by the careful observations of Claparède on Ter. conchilega, where the segments in question are demonstrated to be present from the first.[141]For further details,videgeneral chapter on Nervous System.[142]Reproduction by budding and formation of the sexual products to some extent overlap.
[134]The following classification of the Chætopoda is adopted in the present section:
I.Achæta.(Polygordius).
II.Polychæta.
Sedentaria.
Errantia.
III.Oligochæta.
[135]The observations of Stossich are not thoroughly satisfactory.
[136]According to Willemoes-Suhm, Terebellides strœmii is also characterised by a regular segmentation.
[137]These two rings are at first (Hatschek) not quite closed dorsally, calling to mind the early condition of the Echinoderm larvæ with a præ-oral and post-oral ciliated area.
[138]The structure of the ventral cord in the adult requires further elucidation.
[139]For Semper’s view as to the intercalation of segments in the cephalic region,videnote onp.333.
[140]It has been insisted by Semper (No.355) that certain of the anterior segments, belonging to what he regards as the head region in opposition to the trunk, become interpolated between the trunk and the head. The general evidence, founded on observations of budding, which he brings forward, cannot be discussed here. But the special instance which he cites (founded on Milne-Edwards’s (No.347) observations) of the interpolation of the head segments, bearing the gills, in Terebella appears to me quite unjustified from Milne-Edwards’s own statements; and is clearly shewn to be unfounded by the careful observations of Claparède on Ter. conchilega, where the segments in question are demonstrated to be present from the first.
[141]For further details,videgeneral chapter on Nervous System.
[142]Reproduction by budding and formation of the sexual products to some extent overlap.
The eggs of the Discophora, each enclosed in a delicate membrane, are enveloped in a kind of mucous case formed by a secretion of the integument, which hardens into a capsule or cocoon. In each cocoon there are a limited number of eggs surrounded by albumen. The cocoons are attached to water plants, etc. In Clepsine the embryos leave the cocoon very soon after they get rid of the egg membrane, but in Nephelis they remain within the cocoon for a very much longer period (27‑28 days after hatching). The young of Clepsine, after their liberation, attach themselves to the ventral surface of their parent.
Our knowledge of the development of the Discophora is in a very unsatisfactory state; but sufficient is known to shew that it has very many points in common with that of the Oligochæta, and that the Discophora are therefore closely related to the Chætopoda. In Clepsine there is an epibolic gastrula, and mesoblastic bands like those in Euaxes are also formed. In Nephelis however the segmentation is very abnormal, and the formation of the germinal layers cannot easily be reduced to an invaginate gastrula type, though probably it is modified from such a type. Mesoblastic bands similar to those in the Oligochæta occur in this form also.
The embryology of Clepsine, which will serve as type for the Leeches without jaws (Rhyncobdellidæ), has recently been studied by Whitman (No.365), and that of Nephelis, which willserve as type for the Leeches with jaws (Gnathobdellidæ), has been studied by Bütschli (No.359). The early history of both types is imperfectly known[144].
Formation of the layers.
Clepsine.It is necessary to give a full account of the segmentation of Clepsine, as the formation of the germinal layers would be otherwise unintelligible.
Segmentation commences with the division of the ovum into two unequal spheres by a vertical cleavage passing from the animal to the vegetative pole. By a second vertical cleavage the large segment is divided into two unequal parts, and the small one into two equal parts. Of the four segments so produced three are relatively small, and one, placed at the posterior end, is large. Each of the four segments next gives rise to a small cell at the animal pole. These small cells form the commencement of the epiblast, and, according to Whitman, the mouth is eventually placed in their centre. Such a position for the mouth, at the animal pole, is extremely unusual, and the statements on this head require further confirmation.
Two views of larva of ClepsineFig. 158. Two views of the larva of Clepsine.(After Whitman.)o.oral extremity;m.mouth;pr.germinal streak.A. This figure shews the blastoderm (shaded) with a thickened edge formed by the primitive (i.e.mesoblastic) streaks with the four so-called neuroblasts posteriorly. The vitelline spheres are left without shading.B. represents an embryo in which the blastoderm has enclosed the yolk, and in which the division into segments has taken place. At the hind end are shewn the so-called neuroblasts forming the termination of the germinal streak.
Fig. 158. Two views of the larva of Clepsine.(After Whitman.)
o.oral extremity;m.mouth;pr.germinal streak.
A. This figure shews the blastoderm (shaded) with a thickened edge formed by the primitive (i.e.mesoblastic) streaks with the four so-called neuroblasts posteriorly. The vitelline spheres are left without shading.B. represents an embryo in which the blastoderm has enclosed the yolk, and in which the division into segments has taken place. At the hind end are shewn the so-called neuroblasts forming the termination of the germinal streak.
The posterior large segment now divides into two, one of which is dorsal, and the other and larger ventral. The former I shall call with Whitman the neuroblast, and the latter the mesoblast. The mesoblast very shortly divides again. During the formation of the neuroblast and mesoblast additional epiblastic small cells are added from the three spheres which give rise to three of the primitive epiblast cells, which may now be called the vitelline spheres.
The neuroblast next divides into ten cells, of which the two smaller are soon broken up into epiblastic cells, while the remaining eight arrange themselves in two groups of four each, one group on each side at the posterior border of the epiblastic cap. The two mesoblasts also take up a position on the right and left sides immediately ventral to the four neuroblasts of each side. The neuroblasts and mesoblasts now commence toproliferate at their anterior border, and produce on each side a thickened band of cells underneath the edge of the cap of epiblast cells. Each of these bands is formed of a superficial quadruple[145]row of neuroblasts budded off from the four primary neuroblasts, and a deeper row of mesoblasts. The compound streaks so formed may be called the germinal streaks.
The general appearance of the embryo as seen from the dorsal surface, after the appearance of the two germinal streaks, may be gathered fromfig. 158A. The epiblastic cap in this figure is shaded. The epiblastic cap, accompanied by the germinal streaks, now rapidly extends and encloses the three vitelline spheres by a process equivalent to that of an ordinary epibolic gastrula; but the front and hind ends of the streaks remain practically stationary. Owing to this mode of growth the edges of the epiblastic cap and the germinal streaks meet in a linear fashion along the ventral surface of the embryo (fig. 159, A and B). The germinal streaks first meet anteriorly (B) and their junction is then gradually continued backwards. The process is completed at about the time of hatching.
During the above changes the nuclei of the vitelline spheres pass to the surface and rapidly divide. Eventually, together with part of the protoplasm of the vitelline spheres, they appear to give rise to a layer of hypoblastic cells. This layer encloses the remains of the vitelline spheres, which become the yolk.
Two embryos of ClepsineFig. 159. Two embryos of Clepsine in which the germinal streaks have partially met along the ventral line.(After Robin.)gs.germinal,i.e.mesoblastic streaks.The area covered by epiblast is shaded. The so-called neuroblasts at the end of the germinal streaks are shewn in B.
Fig. 159. Two embryos of Clepsine in which the germinal streaks have partially met along the ventral line.(After Robin.)
gs.germinal,i.e.mesoblastic streaks.
The area covered by epiblast is shaded. The so-called neuroblasts at the end of the germinal streaks are shewn in B.
At the front end of the germinal streaks, in a position corresponding with that of the four original epiblast cells, two depressions appear which coalesce to form the single oral invagination; in the centre of which are formed the mouth and pharynx by a second epiblastic invagination.
The most important point in connection with the above history is the fate of what have been called the germinal streaks. According to Whitman they are composed of two kinds of cells,viz.four rows of smaller superficial cells, which he calls neuroblasts, and, in the later stages at any rate, a row of deeper large cells, which he calls mesoblasts. As to the eventual fate of these cells he states that the neuroblasts uniting together in the median line form the rudiment of the ventral ganglionic chain, while the mesoblasts equally coalesce and give rise to the mesoblast. Such a mode of origin for a ventral ganglionic chain is, so far as I know, without a parallel in the whole animal kingdom; and whatever evidence Whitman may have that the cellsin question really do give rise to the nervous system he has not thought fit to produce it in his paper. He figures a section with the eight neuroblastic cells in the middle ventral line, and in the next stage described the nervous system is divided up into ganglia! The first stage, in which the so-called nervous system has the form of a single row of eight cells, is quite unlike any rudiment of the nervous system such as is usually met with in the Chætopoda, and not a single stage between this and a ganglionated cord is described or figured. Whitman, whose views seem to have been influenced by a peculiar, and in my opinion erroneous, theory of Rauber’s about the relation of the neural groove of Vertebrata to the blastopore, does not seem to be aware that his determination of the fate of his neuroblasts requires any special support.
He quotes the formation of these parts in Euaxes (videpreceding Chapter,p.324) as similar to that in Clepsine. In this comparison it appears to me probable that he may be quite correct, but the result of the comparison would be to shew that the neuroblasts and mesoblasts composed together a mesoblastic band similar to that of the Oligochæta. Till more evidence is brought forward by Whitman or some other observer in support of the view that the so-called neuroblasts have any share in forming the nervous system, they must in my opinion be regarded as probably forming, in conjunction with the mesoblasts, two simple mesoblastic bands. Kowalevsky has moreover briefly stated that he has satisfied himself that the nervous system in Clepsine originates from the epiblast—a statement which certainly could not be brought into harmony with Whitman’s account.
Nephelis.Nephelis will form my type of the Gnathobdellidæ. The segmentation of this form has not yet been thoroughly investigated, but Bütschli’s (No.359) observations are probably the most trustworthy.
The ovum first divides into two, and then into four segments of which two are slightly smaller than the others. Four small cells which form the commencement of the epiblast are now formed. Three of them are derived by budding from the two larger and one of the smaller of the four cells, and the fourth from a subsequent division of one of the larger cells[146]. The three cells which assisted in the formation of the epiblast cells again give rise each to a small cell; and the small cells so formed constitute a layer underneath the epiblast which is the commencement of the hypoblast, while the cells from which they originated form the vitelline spheres. Shortly after the formation of the hypoblast, the large sphere which has hitherto been quiescent divides into two, one of which then gives rise in succession to two small epiblastic elements.
The two large spheres, resulting from the division of the originally quiescent sphere, next divide again on the opposite side of the embryo, and form a layer of epiblast there; so that there is now on one side of the embryo (the ventral according to Robin) a layer of epiblast formedof six cells, and on the opposite side a layer formed of four cells. The two layers meet at the front border of the embryo and between them are placed the three large vitelline spheres. The two patches of epiblast cells now rapidly increase, and gradually spread over the three large vitelline spheres. Except where they meet each other at the front edge they leave uncovered a large part of the margin of the vitelline spheres.
While these changes have been taking place on the exterior, the hypoblast cells have increased in number (additional cells being probably derived from the three large vitelline spheres) and fill up in a column-like fashion a space which is bounded behind by the three vitelline spheres, and in front by the epiblast of the anterior end of the embryo. At the sides of the hypoblast the mesoblast has become established, probably as two lateral bands. The origin of the cells forming it has not yet been determined. The hypoblast cells in the succeeding stage arrange themselves round a central archenteric cavity, and at the same time rapidly increase in size and become filled with a secondary deposit of food-yolk. Shortly afterwards a mouth and thick-walled œsophagus are formed, probably from an epiblastic invagination. The mesoblast now forms two curved lateral bands at the two sides of the body, equivalent to the mesoblastic bands of the Chætopoda. The three vitelline spheres, still largely uncovered by the epiblast, lie at the posterior end of the body. The embryo grows rapidly, especially anteriorly, and the three vitelline spheres become covered by a layer of flattened epiblast cells. Around the œsophagus a cavity traversed by muscular fibres is established. Elsewhere there is no trace of such a cavity. The cephalic region becomes ciliated, and the dorsal part of it, which represents a rudimentary præ-oral lobe, is especially prominent. The cilia of the oral region are continued into the lumen of the œsophagus, and at a later period are prolonged, as in Lumbricus, along the median line of the ventral surface.
The mesoblastic bands would seem from Bütschli’s observations, which receive confirmation from Kleinenberg’s researches on Lumbricus, to be prolonged dorsally to the œsophagus into the cephalic region. Posteriorly they abut on the large vitelline spheres, which were supposed by Kowalevsky to give origin to them, and to play the same part as the large mesoblasts in Lumbricus. It has already been shewn that the function of the large cells in Lumbricus has been exaggerated, and Bütschli denies to them in Nephelis any share in the production of the mesoblast. It seems in fact probable that they are homologous with the three vitelline spheres of Clepsine; and that their primitive function is to give origin to the hypoblast. They are visible for a long time at the hind end of the embryo, but eventually break up into smaller cells, the fate of which is unknown.
The embryo of Hirudo would appear from the researches of Robin to develop in nearly the same way as that of Nephelis. The anterior part is not however ciliated. The three large posterior cells disappear relatively early.
General history of the larva.
The larva of Clepsine, at the time when the mesoblastic bands have met along the ventral line, is represented infig. 158B. It is seen to be already segmented, the process having proceededpari passuwith the ventral coalescence of the mesoblastic bands. The segments are formed from before backwards as in Chætopoda. The dorsal surface is flat and short, and the ventral very convex. The embryo about this time leaves its capsule, and attaches itself to its parent. It rapidly elongates, and the dorsal surface, growing more rapidly than the ventral, becomes at last the more convex. Eventually thirty-three post-oral segments become formed; of which the eight last coalesce to form the posterior sucker.
The general development of the body of Nephelis and Hirudo is nearly the same as that of Clepsine. The embryo passes from a spherical to an oval, and then to a vermiform shape. For full details the reader is referred to Robin’s memoir.
The presence of a well-marked protuberance above the œsophagus, which forms the rudiment of a præ-oral lobe, has already been mentioned as characteristic of the embryo of Nephelis; no such structure is found in Clepsine.
History of the germinal layers and development of organs.
The epiblast.The epiblast is formed of a single layer of cells and early develops a delicate cuticle which is clearly formed quite independently of the egg membrane. It becomes raised into a series of transverse rings which bear no relation to the true somites of the mesoblast.
The nervous system.The nervous system is probably derived from the epiblast, but its origin still requires further investigation. The ventral cord breaks up into a series of ganglia, which at first correspond exactly with the somites of the mesoblast. Of these, four or perhaps three eventually coalesce to form the sub-œsophageal ganglion, and seven or eight become united in the posterior sucker.
It would appear from Bütschli’s statements that the supra-œsophagealganglion arises, as in Oligochæta, independently of the ventral cord.
Mesoblast.It has already been indicated that the mesoblast probably takes its origin both in Nephelis and Clepsine from the two mesoblastic bands which unite in the median ventral line. The further history of these bands is only imperfectly known. They become segmented from before backwards. The somites formed by the segmentation gradually grow upwards and meet in the dorsal line. Septa are formed between the somites probably in the same way as in the Oligochæta.
In Clepsine the mesoblastic bands are stated by Kowalevsky to become split into somatic and splanchnic layers, between which are placed the so-called lateral sinuses. These sinuses form, according to Whitman, a single continuous tube investing the alimentary tract; a tube which differs therefore to a very small extent from the normal body cavity of the Chætopoda. The somatic layer of mesoblast no doubt gives rise to the circular and longitudinal muscular layers of the embryo. The former is stated to appear the earliest, while the latter, as in the Oligochæta, first takes its origin on the ventral side.
A delicate musculature, formed mainly of transverse but also of longitudinal fibres, would appear to be developed independently of the mesoblastic bands in Nephelis and Hirudo (Rathke, Leuckart, Robin, and Bütschli). It develops apparently from certain stellate cells which are found between the walls of the alimentary tract and the skin, and which probably correspond to the system of contractile fibres which pass from the body wall to the alimentary tract through the segmentation cavity in the larva of Chætopoda, various Vermes and Mollusca[147].
The mesoblast, so far as is known, gives rise, in addition to the parts already mentioned, to the excretory organs, generative organs, vascular system, etc.
Excretory organs.There are found in the embryo of Nephelis and Hirudo certain remarkable provisional excretory organs the origin and history of which is not yet fully made out. In Nephelis they appear as one (according to Robin,No.364), or (according to Bütschli,No.359) as two successive pairs ofconvoluted tubes on the dorsal side of the embryo, which are stated by the latter author to develop from the scattered mesoblast cells underneath the skin. At their fullest development they extend, according to Robin, from close to the head to near the ventral sucker. Each of them is U-shaped, with the open end forwards, each limb of the U being formed by two tubes united in front. No external opening has been clearly made out. Semper believed that the tubes were continuous with the three posterior vitelline cells, but this has been shewn not to be the case. Fürbringer[148]is inclined from his own researches to believe that they open laterally. They contain a clear fluid.
In Hirudo, Leuckart (No.362) has described three similar pairs of organs the structure of which he has fully elucidated. They are situated in the posterior part of the body, and each of them commences with an enlargement from which a convoluted tube is continued for some distance backwards; it then turns forwards again and afterwards bends upon itself to open to the exterior. The anterior part is broken up into a kind of labyrinthic network.
The true segmental organs are found in a certain number of the segments and are stated (Whitman) to develop from groups of mesoblast cells. Their origin requires however further investigation.
A double row of colossal cells on each side of the body has been described in Clepsine by Whitman as derived from the mesoblastic plates. These cells (fig. 58B), which he calls segment-cells, lie opposite the walls of the septa. The inner row is stated to be connected with the segmental organs. Their eventual history is unknown, but they are conjectured by Whitman to be the mother cells of the testes.
The alimentary tract.This is formed primitively of two parts—the epiblastic stomodæum—forming mouth, pharynx, and œsophagus, and the hypoblastic mesenteron. The anus is formed very late as a simple perforation immediately dorsal to the posterior sucker.
In Clepsine, where there is an epibolic gastrula, the rudimentof the mesenteron is at first formed of the three vitelline spheres, from the surface of which a true hypoblastic layer enclosing a central yolk mass becomes differentiated, as already described. The mesenteric sack so formed is constricted by the growth of the mesoblastic septa into a series of lobes, while the posterior part forms a narrow and at first very short tube opening by the anus.
The lobed region forms the sacculated stomach of the adult. The sacculations of the stomach by their mode of origin necessarily correspond with the segments. In the adult however the anterior lobe is really double and has two divisions for the two segments it fills, while the posterior lobe, which, as is well known, extends backwards parallel with the rectum, is composed of five segmental sacculations. In connection with the stomodæum a protrusible pharynx is developed.
In Hirudo and Nephelis the mesenteron has from the first a sack-like form. The cells which compose the sack give rise to a secondary deposit of food-yolk. The further changes are practically the same as in Clepsine. In Hirudo the posterior sacculation of the stomach is primitively unpaired. The jaws are formed at about the same time as the eyes as protuberances on the wall of the oral cavity.
Bibliography.
(359)O. Bütschli.“Entwicklungsgeschichtliche Beiträge (Nephelis).”Zeit. f. wiss. Zool.Vol.XXIX.1877.(360)E. Grube.Untersuchungen üb. d. Entwicklung d. Anneliden.Königsberg, 1844.(361)C. K. Hoffmann.“Zur Entwicklungsgeschichte d. Clepsineen.”Niederländ. Archiv f. Zool.Vol.IV.1877.(362)R. Leuckart.Die menschlichen Parasiten (Hirudo),Vol.I.p.686, et seq.(363)H. Rathke.Beit. z. Entwicklungsgesch. d. Hirudineen.Leipzig, 1862.(364)Ch. Robin.Mém. sur le Développement embryogenique des Hirudinées.Paris, 1875.(365)C. O. Whitman. “Embryology of Clepsine.”Quart. J. of Micro. Science,Vol.XVIII.1878.
[Videalso C. Semper (No.355) and Kowalevsky (No.342) for isolated observations.]
[143]The Discophora are divided into the following groups:I.Rhyncobdellidæ.II.Gnathobdellidæ.III.Branchiobdellidæ.[144]Hoffmann’s account (No.36) is so different from that of other observers that I have been unable to make any use of it.[145]According to Robin it is more usual for there to be only a triple row of primary neuroblasts.[146]Doubts have been cast by Whitman on the above account of the origin of the four epiblast cells.[147]According to Robin this system of muscles becomes gradually strengthened and converted into the permanent system. Rathke on the other hand states that it is provisional, and that it is replaced by the muscles developed from the mesoblastic somites. It is possible to suppose that it may really become incorporated in the latter system.[148]Morphologisches Jahrbuch,Vol.IV.p.676. He further speaks of the tube as“feinverzweigt u. netzförmig verästelt,”but whether from his own observations is not clear.
[143]The Discophora are divided into the following groups:
I.Rhyncobdellidæ.
II.Gnathobdellidæ.
III.Branchiobdellidæ.
[144]Hoffmann’s account (No.36) is so different from that of other observers that I have been unable to make any use of it.
[145]According to Robin it is more usual for there to be only a triple row of primary neuroblasts.
[146]Doubts have been cast by Whitman on the above account of the origin of the four epiblast cells.
[147]According to Robin this system of muscles becomes gradually strengthened and converted into the permanent system. Rathke on the other hand states that it is provisional, and that it is replaced by the muscles developed from the mesoblastic somites. It is possible to suppose that it may really become incorporated in the latter system.
[148]Morphologisches Jahrbuch,Vol.IV.p.676. He further speaks of the tube as“feinverzweigt u. netzförmig verästelt,”but whether from his own observations is not clear.
It is convenient for the purposes of embryology to divide the Gephyrea into two groups,viz.(1) Gephyrea nuda or true Gephyrea; and (2) Gephyrea tubicola formed by the genus Phoronis.
Gephyrea nuda.
Segmentation and formation of the layers.
An embolic or epibolic gastrula is characteristic of the Gephyrea, and the blastopore appears, in some cases at any rate (Phascolosoma, Thalassema), to become the mouth.
Bonellia.In Bonellia (Spengel,No.370) the segmentation is unequal but complete, and, as in many Molluscs etc., the ovum exhibits before its commencement a distinction into a protoplasmic and a yoke pole. The ovum first divides into four equal segments, each of them formed of the same constituents as the original ovum. At the animal pole four small cells, entirely formed of protoplasm, are next formed by an equatorial furrow. They soon place themselves in the intervals between the large spheres. Four small cells are again budded off from the large spheres and the eight small cells then divide. By a further continuation of the division of the existing small cells, and the formation of fresh ones from the large spheres, a layer of smallcells is eventually formed, which completely envelops the four large spheres except for a small blastopore at the vegetative pole of the ovum (fig. 160A). The large spheres continue to give rise to smaller cells which however no longer take a superficial position but lie within the layer of small cells, and give rise to the hypoblast (fig. 160B). The small cells become the epiblast, and at the blastopore they curl inwards (fig. 160B) and give rise to a layer of cells, which appears to extend as an unbroken sheet between the epiblast and hypoblast, and to form the mesoblast. The blastopore now closes up, but its position in relation to the parts of the embryo has not been made out.
Epibolic gastrula of BonelliaFig. 160. Epibolic gastrula of Bonellia.(After Spengel.)A. Stage when the four hypoblast cells are nearly enclosed.B. Stage after the formation of the mesoblast has commenced by an infolding of the lips of the blastopore.ep.epiblast;me.mesoblast;bl.blastopore.
Fig. 160. Epibolic gastrula of Bonellia.(After Spengel.)
A. Stage when the four hypoblast cells are nearly enclosed.B. Stage after the formation of the mesoblast has commenced by an infolding of the lips of the blastopore.
ep.epiblast;me.mesoblast;bl.blastopore.
In Phascolosoma (Selenka,No.369) the ovum, enclosed in a porous zona radiata, divides into two unequal spheres, of which the smaller next divides into two and then into four. An invagination takes place which is intermediate between the embolic and the epibolic types. The small cells, the number of which is increased by additions from the large sphere, divide, and grow round the large sphere. The latter in the meantime also divides, and the cells produced from it form on the one hand a small sack which opens by the blastopore, and on the other they fill up the segmentation cavity, and become the mesoblast and blood corpuscles. The blastopore becomes the permanent mouth.
Larval forms and development of organs.
Amongst the Gephyrea armata the larva has as a rule (Thalassema, Echiurus) the characters of a trochosphere, and closely approaches the typical form characteristic of the larva of Polygordius, often known as Lovén’s larva. In Bonellia this larval form is less perfectly preserved.
Echiurus.In Echiurus (Salensky,No.368) the youngest known larva has all the typical trochosphere characters (fig. 161). It is covered with cilia and divided into a præ-oral lobe and post-oral region of nearly equal dimensions. There is a double ciliated ring which separates the two sections of the body as in the larva of Polygordius: the mouth (m) opens between its two elements. The alimentary canal is divided into a stomodæum with a ventral opening, a large stomach, and a short intestine opening by a terminal anus (an). Connecting the œsophagus with the apex of the præ-oral lobe is the usual contractile band, and at the insertion of this band is a thickening of the epiblast which probably represents the rudiment of the supra-œsophageal ganglion. A ventral nerve cord is stated by Salensky to be present, but his observations on this point are not quite satisfactory.
Larva of EchiurusFig. 161. Larva of Echiurus.(After Salensky.)m.mouth;an.anus;sg.supra-œsophageal ganglion (?).
Fig. 161. Larva of Echiurus.(After Salensky.)
m.mouth;an.anus;sg.supra-œsophageal ganglion (?).
The metamorphosis is accompanied by the loss of swimming power, and consists in the enlargement of the post-oral portion of the trunk, and in the simultaneous reduction of the præ-oral lobe, which remains however permanently as the cylindrical proboscis. A groove which terminates posteriorly at the mouth is very early formed on its ventral side. The ciliated rings gradually disappear during the metamorphosis.
Of the further externalchanges the most important are (1) the early appearance round the anal end of the body of a ring of bristles; and (2) the appearance of a pair of ventral setæ in the anterior part of the body. The anterior ring of bristles characteristic of the adult Echiurus does not appear till a late period.
Of the internal changes the earliest is the formation of the anal respiratory sacks. With the growth of the posterior part of the trunk the intestine elongates, and becomes coiled.
Bonellia.The embryo of Bonellia, while still within the egg, retains a spherical form and acquires an equatorial band of cilia, behind which a second narrower band is soon established, while in front of the first one a pair of eye-spots becomes formed (fig. 162A). The embryo on becoming hatched rapidly elongates, while at the same time it becomes dorso-ventrally flattened and acquires a complete coating of cilia (fig. 162B). According to Spengel it resembles at this time in its form and habits a rhabdocœlous Turbellarian. The anterior part is however somewhat swollen and presents an indication of a præ-oral lobe.
Three stages in the development of BonelliaFig. 162. Three stages in the development of Bonellia.(After Spengel.)A. Larva with two ciliated bands and two eye-spots.B. Ripe larva from the dorsal surface.C. Young female Bonellia from the side.al.alimentary tract;m.mouth;se.provisional excretory tube;s.ventral hook;an.v.anal vesicle.
Fig. 162. Three stages in the development of Bonellia.(After Spengel.)
A. Larva with two ciliated bands and two eye-spots.B. Ripe larva from the dorsal surface.C. Young female Bonellia from the side.
al.alimentary tract;m.mouth;se.provisional excretory tube;s.ventral hook;an.v.anal vesicle.
During the above changes important advances are made in the formation of the organs from the embryonic layers.
The epiblast acquires a superficial cuticula, which is perhaps directly derived from the vitelline membrane. The nervous system is also formed, probably from the epiblast. The band-like supra-œsophageal ganglion is the first part of the nervous system formed, and appears to be undoubtedly derived from the epiblast. The ventral cord arises somewhat later, but the first stages in its development have not been satisfactorily traced. It is continuous with the supra-œsophageal band which completely girths the œsophagus without exhibiting any special dorsal enlargement. After the ventral cord has become completely separated from the epiblast a central fibrous mass becomes differentiated in it, while the lateral parts are composed of ganglion cells. In the arrangement of its cells it presents indications of being composed of two lateral halves. It is, however, without ganglionic swellings.
The mesoblast, though at first very thin, soon exhibits a differentiation into a splanchnic and somatic layer—though the two do not become distinctly separated by a body cavity. The somatic layer rapidly becomes thicker, and enlarges laterally to form two bands united dorsally and ventrally by narrow, thinner bands. The outermost parts of each of these bands become differentiated into an external circular and an internal longitudinal layer of muscles. In the præ-oral lobe the mesoblast assumes a somewhat vacuolated character.
The hypoblast cells form a complete layer round the four yolk cells from which they arise (fig. 162B,al), but at first no alimentary lumen is developed. The œsophagus appears during this period as an, at first solid, but subsequently hollow, outgrowth of the hypoblast towards the epiblast.s
The metamorphosis of the larva into the adult female Bonellia commences with the conversion of many of the indifferent mesoblast cells into blood corpuscles, and the introduction into the body cavity of a large amount of fluid, which separates the splanchnic and somatic layers of mesoblast. The fluid is believed by Spengel to be sea-water, introduced by two anal pouches, the development of which is described below.
The body cavity is lined by a peritoneum, and very soon distinct vessels, formed by folds of the peritoneum, become established. Of these there are three trunks, two lateral and a median in the præ-oral lobe (proboscis), and in the body a ventral trunk above the nerve cord, and an intestinal trunk opening anteriorly into the ventral one. The vessels appear to communicate with the body cavity.
In the course of the above changes the two ciliated bandsdisappear, the hinder one first. The cilia covering the general surface become atrophied, with the exception of those on the ventral side of the præ-oral lobe. The latter structure becomes more prominent; the stellate mesoblast cells, which fill up its interior, become contractile, and it gives rise to the proboscis (fig. 162C).
At the point where the œsophageal protuberance joined the epiblast at a previous stage the mouth becomes established (fig. 162C,m), and though it is formed subsequently to the atrophy of the anterior ciliated band, yet there is evidence that it is potentially situated behind this band. The lumen of the alimentary canal becomes established by the absorption of the remains of the four central cells. The anus is formed on the ventral side of the posterior end of the body, and close to it the pouches already noticed grow out from the hindermost part of the alimentary tract (fig. 162C,an.v). They are at first simple blind pouches, but subsequently open into the body cavity[150]. They become the anal pouches of the adult. There is present when the mouth is first formed a peculiar process of the alimentary tract projecting into the præ-oral lobe, which appears to atrophy shortly afterwards.
After the formation of the mouth, there are formed on the ventral side of and slightly behind it (1) anteriorly a pair of tubes, which appear to be provisional excretory organs and soon disappear (fig. 162C,sc); and (2) behind them a pair of bristles (s) which remain in the adult. The formation of the permanent excretory (?) organ (oviduct and uterus) has not been followed out. The ovary appears very early as a differentiation of the epithelium lining the ventral vessel.
The larvæ, which become the minute parasitic males, undergo a very different and far less complete metamorphosis than those which become females. They attach themselves to the proboscis of an adult female, and lose their ciliated bands. Germinal cells make their appearance in the mesoblast, which form spherical masses, and, like the germinal balls in the female ovary, consist of a central cell, and an epithelium around it. The central cell becomes very large, while the peripheral cells give rise to the spermatozoa. A body cavity becomes developed in the larvæ, into which the spermatic balls are dehisced. Neither mouth nor anus is formed. The further changes have not been followed out.
The larval males make their way into the œsophagus of the female, where they no doubt live for some time, and probably become mature, though the seminal pouch of the adult is not found in many of the males living in the œsophagus. When mature the males leave the œsophagus, and pass into the uterus.
Phascolosoma.Cilia appear in Phascolosoma (Selenka,No.369) while the ovum is still segmenting. After segmentation they form a definite band immediatelybehindthe mouth, which divides the larva into two hemispheres—a præ-oral and a post-oral. A præ-oral band of cilia is soon formed close to the post-oral band, and at the apex of the præ-oral lobe a tuft of cilia also appears.
The larva has now the characters of a trochosphere, but differs from the typical trochosphere in the post-oral part of the ciliated equatorial ring being more important than the præ-oral, and in the absence of an anus.
The metamorphosis commences very early. The trunk rapidly elongates, and the præ-oral lobe becomes relatively less and less conspicuous. The zona radiata becomes the larval cuticle.
Three pairs of bristles are formed on the trunk, of which the posterior pair appears first, then the anterior, and finally the middle pair: an order of succession which clearly proves they can have no connection with a true segmentation.
The tentacles become developedbetweenthe two parts of the ciliated ring, and finally the præ-oral lobe, unlike what takes place in the Gephyrea armata, nearly completely vanishes.
The anus appears fairly late on the dorsal surface, and the ventral nerve cord is established as an unganglionated thickening of the ventral epiblast.
Gephyrea tubicola.
The larva of Phoronis was known as Actinotrocha long before its connection with Phoronis was established by Kowalevsky (No.372). There is a complete segmentation leading to the formation of a blastosphere, which is followed by an invagination, the opening of which is said by Kowalevsky to remain asthe mouth[151]. It is at first terminal, but on the development of a large præ-oral lobe it assumes a ventral position. The anus is formed at a later period at the posterior end of the body.