[118]The classification of the Polyzoa adopted in this chapter is shewn in the subjoined table:I.Entoprocta.II.Ectoprocta.1.Gymnolæmataa.Chilostomata.b.Ctenostomata.c.Cyclostomata.2.Phylactolæmata.3.Podostomata(Rhabdopleura).[119]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.[120]Lankester. “Remarks on the Affinities of Rhabdopleura.”Quart. J. of Micro. Science,Vol.XIV.1874.[121]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.[122]The note (No.307) refers in the first instance to the changes in the larvæ of the Chilostomata, but the similarity of the larvæ 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.[123]The interpretation of the larvæ given in the text must be regarded as somewhat tentative. The opacity of the free larvæ is very great, and almost every one of the numerous authors who have worked on these larvæ have arrived at different conclusions, as to the physiological significance of the various parts.[124]Barrois himself held the opposite view in his earlier memoir, and other observers have done the same.[125]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.[126]Lankester. “Remarks on the affinities of Rhabdopleura.”Quart. J. of Micro. Science,Vol.XIV.1874.[127]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 diagrammatically put in black for greater distinctness.
[118]The classification of the Polyzoa adopted in this chapter is shewn in the subjoined table:
I.Entoprocta.
II.Ectoprocta.
1.Gymnolæmata
a.Chilostomata.
b.Ctenostomata.
c.Cyclostomata.
2.Phylactolæmata.
3.Podostomata
(Rhabdopleura).
[119]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.
[120]Lankester. “Remarks on the Affinities of Rhabdopleura.”Quart. J. of Micro. Science,Vol.XIV.1874.
[121]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.
[122]The note (No.307) refers in the first instance to the changes in the larvæ of the Chilostomata, but the similarity of the larvæ 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.
[123]The interpretation of the larvæ given in the text must be regarded as somewhat tentative. The opacity of the free larvæ is very great, and almost every one of the numerous authors who have worked on these larvæ have arrived at different conclusions, as to the physiological significance of the various parts.
[124]Barrois himself held the opposite view in his earlier memoir, and other observers have done the same.
[125]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.
[126]Lankester. “Remarks on the affinities of Rhabdopleura.”Quart. J. of Micro. Science,Vol.XIV.1874.
[127]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 diagrammatically put in black for greater distinctness.
The observations which have been made on the developmental history of the Brachiopoda have thrown very considerable light on the systematic position of this somewhat isolated group.
Development of the Layers.
For our knowledge of the early stages in the development of the Brachiopoda we are almost entirely indebted to Kowalevsky[129](No.326). His researches extend to four forms, Argiope, Terebratula, Terebratulina, and Thecidium. The early development of the first three of these takes place on one plan, and that of Thecidium on a second plan.
In Argiope, which may be taken as typical of the first group, the ova are transported into the oviducts (segmental organs) where they undergo their early development. The segmentation leads to the formation of a blastosphere, which then becomes a gastrula by invagination. The blastopore gradually narrows, and finally closes, while at the same time the archenteric cavity(fig. 135A) becomes divided into three lobes, a median (me) and two lateral (pv). These lobes next become completely separated, and the middle one forms the mesenteron, while the two lateral ones give rise to the body cavity, their outer walls forming the somatic mesoblast, and their inner the splanchnic (fig. 135B). The embryo now elongates, and becomes divided into three successive segments (fig. 135B), which are usually, though on insufficient grounds (videThecidium), regarded as equivalent to the segments of the Chætopoda. The alimentary tract is not continued into the hindermost of them.
Two stages of ArgiopeFig. 135. Two stages in the development of Argiope.(After Kowalevsky.)A. Late gastrula stage.B. Stage after the larva has become divided into three segments.bl.blastopore;me.mesenteron;pv.body cavity;b.temporary bristles.
Fig. 135. Two stages in the development of Argiope.(After Kowalevsky.)
A. Late gastrula stage.B. Stage after the larva has become divided into three segments.
bl.blastopore;me.mesenteron;pv.body cavity;b.temporary bristles.
In Thecidium the ova are very large, and development takes place in a special incubatory pouch in the ventral valve. The embryos are attached by suspenders to the two cirri of the arms which immediately adjoin the mouth. There is a nearly regular segmentation, and a very small segmentation cavity is developed. There is no invagination; but cells are budded off from the walls of the blastosphere, which soon form a solid central mass, enclosed by an external layer—the epiblast. In this central mass three cavities are developed, which constitute the mesenteron and the two halves of the body cavity. Around these cavities distinct walls become differentiated. The body (Lacaze Duthiers,No.327) soon after becomes divided into two segments, of which the posterior is the smaller. The hinder part of the large anterior segment next becomes constricted off as a fresh segment, and subsequently the remaining part becomes divided into two, of which the anterior is the smallest. The embryo thus becomes divided into four segments, of which the two foremost appear (?) together to correspond to the cephalic segment of Argiope; but these segments are formed not, as in Chætopoda and other truly segmented forms, by the addition of fresh segments between the last-formed segment andthe unsegmented end of the body, but by the interpolation of fresh segments at the cephalic end of the body as in Cestodes; so that the hindermost segment is the oldest. Assuming the correctness[130]of Lacaze Duthiers’ observations, the mode of formation of these segments appears to me to render it probable that they are not identical with the segments of a Chætopod. A suspender is attached to the front end of each embryo. Before the four segments are established the whole embryo is covered with cilia[131], and two and then four rudimentary eyes are developed on the anterior segment of the body.
The history of the Larva and the development of the organs of the Adult.
Larva of ArgiopeFig. 136. Larva of Argiope.(From Gegenbaur, after Kowalevsky.)m.mantle;b.setæ;d.archenteron.
Fig. 136. Larva of Argiope.(From Gegenbaur, after Kowalevsky.)
m.mantle;b.setæ;d.archenteron.
Articulata.The observations of Kowalevsky and Morse have given us a fairly complete history of the larval metamorphosis of some of the Articulata, while some of the later larval stages in the history of the Inarticulata have been made known to us from the researches of Fritz Müller, Brooks, etc. The embryo of Argiope, which may be taken as the type for the Articulata, was left (fig. 135B) as a three lobed organism with a closed mesenteron and a body cavity divided into two lateral compartments. On the middle segment of the body dorsal and ventral folds, destined to form the mantle lobes, make their appearance, and on the latter two pairs of bundles of setæ are present (fig. 135B). The setæ together with the mantle folds grow greatly, and the setæ resemble in appearance the provisional setæ of many Chætopods (fig. 152). On the hinder border of the mantle cilia make their appearance. The anterior or cephalic segment assumes a somewhat umbrella-like form, and round its edge is a circletof long cilia, while elsewhere it is provided with a coating of short cilia. Two pairs of eyes also arise on its anterior surface (fig. 136).
Two stages of ArgiopeFig. 137. Two stages in the development of Argiope, shewing the folds of the mantle growing over the cephalic lobe.(After Kowalevsky.)m.mantle fold;me.mesenteron;pd.peduncle;b.provisional setæ.
Fig. 137. Two stages in the development of Argiope, shewing the folds of the mantle growing over the cephalic lobe.(After Kowalevsky.)
m.mantle fold;me.mesenteron;pd.peduncle;b.provisional setæ.
After swimming about for some time the larva becomes fixed by its hind lobe, and becomes gradually transformed into the adult. The hind lobe itself becomes the peduncle. After attachment the mantle lobes bend forward (fig. 137A,m), and enclose the cephalic lobe. The valves of the shell are formed on their outer surface as two delicate chitinous plates (fig. 137B). At a somewhat later stage the provisional bristles are thrown off, and are eventually replaced by permanent setæ round the edge of the mantle. The cephalic lobe becomes located in the dorsal valve of the shell, and the mouth is formed near the apex of the cephalic lobe immediately ventral to the eye-spots, by an epiblastic invagination. The permanent muscles are formed out of the muscles already present in the embryo.
Around the mouth there arises a ring of tentacles, verypossibly derived from the ciliated ring visible infig. 136[132]. The ring of tentacles is placed obliquely, and the mouth is situated near its ventral side. The tentacles appear to form a post-oral circlet, like that of Phoronis (Actinotrocha): they gradually increase in number as the larva grows older.
Advanced embryo of LingulaFig. 138. Diagram of a longitudinal vertical section of an advanced embryo of Lingula.(After Brooks.)a.end of valves;b.thickened margin of mantle;c.mantle;d.dorsal median tentacle;e.lophophore;f.lip;g.mouth;h.mantle cavity;i.body cavity;k.wall of œsophagus;l.œsophagus;m.hepatic chamber of stomach;n.intestinal chamber of stomach;o.intestine;q.ventral ganglion;r.posterior muscle;s.dorsal valve of shell;t.ventral valve of shell.
Fig. 138. Diagram of a longitudinal vertical section of an advanced embryo of Lingula.(After Brooks.)
a.end of valves;b.thickened margin of mantle;c.mantle;d.dorsal median tentacle;e.lophophore;f.lip;g.mouth;h.mantle cavity;i.body cavity;k.wall of œsophagus;l.œsophagus;m.hepatic chamber of stomach;n.intestinal chamber of stomach;o.intestine;q.ventral ganglion;r.posterior muscle;s.dorsal valve of shell;t.ventral valve of shell.
Some of the later stages in the development of the Terebratulidæ have been made known to us by the observations of Morse (No.328‑9) on Terebratulina septentrionalis.
The most interesting point in Morse’s observations on the later stages is the description of the gradual conversion of the disc bearing the circlet of tentacles into the arms of the adult. The tentacles, six in number, first form a ring round the edge of a disc springing from the dorsal lobe of the mantle; in their centre is the mouth. In the later stages calcareous spicula become developed on the tentacles. When the embryo is far advanced the tentacles begin to assume a horse-shoe arrangement, which bears a striking, though probably accidental, resemblance to that of the tentacles on the lophophore of the fresh-water Polyzoa. The disc bearing the tentacles is prolonged anteriorly into two processes, the free ends of the future arms. By this change of shape in the disc the tentacles form two rows, one on the anterior and one on the posterior border of the disc, and eventually become the cirri of the arms. The mouth is placed between the two rows of tentacles, where the two arms of the lophophore meet behind. The position of the mouth was the original centre of the ring of tentacles before they became pulled out into a horse-shoe form. In front of the mouth is a lip. The arms grow greatly in length in the adult Terebratulina. In Thecidium the oral disc retains the horse-shoe form, while in Argiope the embryonic circular arrangement of the tentacles is only interfered with by the appearance of marginal sinuations.
The shell is deposited as to chitinous plates, which subsequently become calcified. It undergoes in the different genera great changes of form during its growth.
With reference to the larval stages of other Articulata, a few points may be noted.
The three-lobed larva of Terebratulina septentrionalis is provided with a special tuft of cilia at the apex of the front lobe. The arms appear to originate, in Terebratulina caput serpentis, as two processes at the sides of the mouth, on which the tentacles are formed.
Provisional setæ do not appear to be formed in the lobed embryos of Thecidium and Terebratulina, but they appear at a later stage at the edge of the mantle in the latter form. The third lobe of Thecidium gives rise to the dorsal and ventral mantle lobes.
Inarticulata.The youngest stages in the development of the Inarticulata are not known, and in the earliest stages observed the shell is already developed. The young larvæ with shells differ however from those of the Articulata in the fact that they are free-swimming, and that the peduncle is not developed.
The larva of Discina radiata has been described by Fritz Müller (No.331). It resembles generally a larva of the Articulata shortly after the tentacles have become developed. Five pairs of long provisional setæ are present, of which all but the hindermost are seated on the ventral lobe of the mantle. Shorter setæ are also lodged on the edge of the dorsal lobe. The mouth is placed on the ventral side of a protrusible oral lobe. It is imperfectly surrounded by four pairs of tentacles, which form a swimming apparatus.
A fuller history of the development of Lingula has been recently supplied by Brooks (No.325). The youngest larva is enveloped in two nearly similar plate-like valves, covering the two mantle lobes. The mouth is placed at the centre of a disc, attached to the dorsal valve, on the margin of which is a ring of ciliated tentacles. The general position of the disc and its relations may be gathered fromfig. 138, which represents a diagrammatic longitudinal vertical section of the embryo.
With the growth of the embryo the tentacles increase in number, the new pairs being always added between the odd dorsal tentacle and the next pair. There is an axial cavity in the tentacles which, unlike the cavity in the tentacles of the Polyzoa, does not communicate with the perivisceral cavity. As the tentacles increase in number, the lateral parts of the tentacular disc grow out into the two lateral arms of the adult, while the dorsal margin forms the median coiled arm. These changes are not effected till the larva has become fixed.
The attachment of the larva was not observed; but the peduncle, ofwhich there is no trace in the young stages, grows out as a simple prolongation of the hinder end of the body while the larva is still free. It had already reached a very great length in the youngest fixed larva observed.
Development of Organs.
The alimentary tract after the obliteration of the blastopore forms a closed sack, which becomes subsequently placed in communication with the exterior by the stomodæal invagination. The liver is formed as a pair of dorsal outgrowths of the mesenteron. From Brooks’ observations on Lingula it would appear that the primitive mesenteron forms the stomach of the adult only, and that the intestine grows out from this as a solid process: this eventually meets the skin, and here the anus is formed. In the Articulata the mesenteron is aproctous.
The origin of the body cavity as paired archenteric diverticula has already been described. Its somatic wall becomes in Lingula ciliated, and its cavity filled with a corpusculated fluid, as in many Chætopods. It is eventually prolonged into the dorsal and ventral mantle lobes as a pair of horn-like prolongations into each lobe, which communicate with the body cavity by large ciliated openings. Some incomplete observations of Brooks on the development of the nervous system in Lingula shew that it arises in the embryo as a ring round the œsophagus with a ventral sub-œsophageal (fig. 138q), and two lateral ganglia, and two dorsal otocysts. The ventral ganglion is formed as a thickening of the epiblast, with which it remains in continuity for life. The remainder of the ring grows out from the ventral ganglion as two cords, which gradually meet on the dorsal side of the œsophagus.
General observations on the Affinity of the Brachiopoda.
The larva of Argiope, as has been noticed by many observers, has undoubtedly very close affinities with the Chætopoda. It resembles, in fact, a mesotrochal larval Chætopod with provisional setæ (videChapter on Chætopoda). Lacaze Duthiers’ observations point to the lobes of the larva not being true segments, and certainly the mesoblast does not in the embryo become segmented as it ought to do were these lobes true segments. If this view is correct the larva is to be compared to an unsegmented Chætopod larva. In Rhynchonella, however, indications of two segments are afforded in the adult in the two pairs of segmental organs.
Though the larval Brachiopod resembles a mesotrochal Chætopod larva,it does not appear to resemble the trochosphere larvæ so far described, or the more typical larvæ of the Chætopoda, in that the ring of tentacles, which is probably, as already mentioned, derived from the ciliated ring shewn infig. 137, ispost-oral, and notpræ-oral. The ring of tentacles is like the ring in Actinotrocha (the larva of Phoronis) amongst the Gephyrea. Although there is no doubt a striking resemblance between the tentacular disc of a larval Brachiopod and the lophophore of a Polyzoon, which has been pointed out by Lankester, Morse, Brooks, etc., their homology is rendered, to my mind, very doubtful (1) by the fact that the lophophore is præ-oral in Polyzoa[133]and post-oral in Brachiopoda; and (2) by the fact that the concave side of the lophophore is turned in nearly opposite directions in the two forms. In Brachiopods it is turned dorsalwards, and in phylactolæmatous Polyzoa ventralwards.
The view of Morse, that the Brachiopoda are degraded tubicolous Chætopods, is not so far supported by any definite embryological facts. The development of the tentacular ring as well as its innervation from the sub-œsophageal ganglion prohibit us, as has been pointed out by Gegenbaur, from comparing it with the tentacles of tubicolous Chætopoda.
Bibliography.
(325)W. K. Brooks. “Development of Lingula.”Chesapeake Zoological Laboratory, Scientific Results of the Session of 1878.Baltimore, J. Murphy andCo.(326)A. Kowalevsky. “Development of the Brachiopoda.”Protocol of the First Session of the United Sections of Anatomy, Physiology, and Comparative Anatomy at the Meeting of Russian Naturalists in Kasan, 1873. (Russian.)(327)H. Lacaze Duthiers.“Histoire de la Thécidie.”Ann. Scien. Nat. etc.Ser. 4, Vol.XV.1861.(328)Morse. “On the Early Stages of Terebratulina septentrionalis.”Mem. Boston Soc. Nat. History,Vol.II.1869, alsoAnn. & Mag. of Nat. Hist., Series 4,Vol.VIII.1871.(329) —— “On the Embryology of Terebratulina.”Mem. Boston Soc. Nat. History,Vol.III., 1873.(330) —— “On the Systematic Position of the Brachiopoda.”Proceedings of the Boston Soc. of Nat. Hist., 1873.(331)Fritz Müller.“Beschreibung einer Brachiopoden Larve.”Müller’sArchiv, 1860.
[128]The classification of the Brachiopoda adopted in the present chapter is shewn in the subjoined table:I.Articulata.a.Rhynchonellidæ.b.Terebratulidæ.II.Inarticulata.a.Lingulidæ.b.Craniadæ.c.Discinidæ.[129]Kowalevsky’s Memoir is unfortunately written in Russian. The account in the text is derived from an inspection of his figures, and from an abstract in Hoffmann and Schwalbe’sJahresberichtefor 1873.[130]It should be stated that it is by no means clear from Kowalevsky’s figures that he agrees with Lacaze Duthiers as to the succession of the segments.[131]Kowalevsky in his figures leaves the penultimate lobe unciliated.[132]In the abstract in Hoffman and Schwalbe Kowalevsky is made to state that the tentacles spring from the border of the mantle. This can hardly be a correct account of what he states, since it does not fit in with the adult anatomy of the parts. The figures he gives might lead to the supposition that they sprang from the edge of the cephalic lobe, or perhaps from the dorsal lobe of the mantle.[133]For the ectoproctous Polyzoa it might be held that the ciliated ring of tentacles is post-oral, but the facts of development recorded in the previous chapter appear to me to shew that this view is untenable.
[128]The classification of the Brachiopoda adopted in the present chapter is shewn in the subjoined table:
I.Articulata.
a.Rhynchonellidæ.
b.Terebratulidæ.
II.Inarticulata.
a.Lingulidæ.
b.Craniadæ.
c.Discinidæ.
[129]Kowalevsky’s Memoir is unfortunately written in Russian. The account in the text is derived from an inspection of his figures, and from an abstract in Hoffmann and Schwalbe’sJahresberichtefor 1873.
[130]It should be stated that it is by no means clear from Kowalevsky’s figures that he agrees with Lacaze Duthiers as to the succession of the segments.
[131]Kowalevsky in his figures leaves the penultimate lobe unciliated.
[132]In the abstract in Hoffman and Schwalbe Kowalevsky is made to state that the tentacles spring from the border of the mantle. This can hardly be a correct account of what he states, since it does not fit in with the adult anatomy of the parts. The figures he gives might lead to the supposition that they sprang from the edge of the cephalic lobe, or perhaps from the dorsal lobe of the mantle.
[133]For the ectoproctous Polyzoa it might be held that the ciliated ring of tentacles is post-oral, but the facts of development recorded in the previous chapter appear to me to shew that this view is untenable.
Formation of the Germinal Layers.
Most Chætopoda deposit their eggs before development. The Oligochæta lay them in peculiar cocoons or sacks formed by a secretion of the integument. Some marine Polychæta carry them about during their development. Autolytus cornutus has a special sack on the ventral surface in which they are hatched. In Spirorbis Pagenstecheri they develop inside the opercular tentacle, and in Spirorbis spirillum inside the tube of the parent.
A few forms (e.g.Eunice sanguinea, Syllis vivipara, Nereis diversicolor) are viviparous.
Perhaps the most primitive type of Chætopod development so far observed is that of Serpula (Stossich,No.357)[135]. There is a regular segmentation resulting in the formation of a blastosphere with a central segmentation cavity. An invagination of the normal type now ensues. The blastopore soon narrows to become the permanent anus, while the invaginated hypoblast forms a small prominence with an imperfectly developed lumen, whichdoes not nearly fill up the segmentation cavity(fig. 139A). The embryo, which has in the meantime become completelycovered with cilia, now assumes more or less the form of a cone, at the apex of which is the anus, while the base forms the rudiment of a large præ-oral lobe. The alimentary sack grows forwards and then bends upon itself nearly at right angles, and meets a stomodæal invagination from the ventral side some way from the front end of the body.
Two stages in the development of SerpulaFig. 139. Two stages in the development of Serpula.(After Stossich.)m.mouth;an.anus;al.archenteron.
Fig. 139. Two stages in the development of Serpula.(After Stossich.)
m.mouth;an.anus;al.archenteron.
The alimentary canal soon differentiates itself into three regions (1) œsophagus, (2) stomach, and (3) intestine. With these changes the larva, which in the meantime becomes hatched, assumes the characters of a typical Annelid larva (fig. 139B). In front is a large præ-oral lobe, at the sides of which the eye-spots soon appear. The primitive segmentation cavity remains as a wide space between the curved alimentary tract and the body walls, and becomes traversed by muscular fibres passing between the two. The original chorion appears to serve as cuticle, and is perforated by the cilia.
The further changes in this larval form do not present features of general importance. A peculiar vesicle, which in anomalous cases is double, is formed near the anus. If it were shewn to occur widely amongst Chætopoda, it might be perhaps regarded as homologous with the anal vesicles of the Gephyrea.
Serpula is one of the few Chætopoda at present known inwhich the segmentation is quite regular[136]. In other forms it is more or less unequal. The formation of the germinal layers has been far more fully studied in the Oligochæta than in the Polychæta, and though unfortunately the development is much abbreviated in the former group, they nevertheless have to serve as our type; and unless the contrary is indicated the statements in the remainder of the section apply to the Oligochæta. The segmentation is nearly regular in Lumbricus agricola (Kowalevsky) and results in the formation of a flattened blastosphere, one of the sides of which is hypoblastic and the other epiblastic, the hypoblast cells being easily distinguished from the epiblast cells by their clearer aspect. An invagination takes place, in the course of which the hypoblast becomes enclosed by the epiblast, and a somewhat cylindrical two-layered gastrula is formed. The opening of this gastrula at first extends over the whole of what becomes the ventral surface of the future worm, but gradually narrows to a small pore—the permanent mouth—near the front end. The central cavity of the gastrula is lined by hypoblast cells, but the oral opening, which leads by a narrow passage into the gastric cavity, is lined by epiblast cells.
The segmentation of Lumbricus trapezoides (Kleinenberg,No.341), and of Criodrilus (Hatschek,No.339), is more unequal and more irregular than that of Lumbricus agricola, and there is an invagination which is intermediate between the embolic and epibolic types.
The segmentation of Lumbricus trapezoides is especially remarkable. It is strangely irregular and at one period the segmentation cavity communicates by a pore with the exterior. Before the completion of the gastrula stage the ovum becomes partially divided into two halves, each of which gives rise to a complete embryo. The two embryos are at first united by an epiblast cord which connects their necks (fig. 141A), but this cord is very early ruptured, and the two embryos then become quite independent. Some of the peculiarities of the segmentation may no doubt be explained by this remarkable embryonic fission.
The gastrula opening in both Lumbricus trapezoides and Criodrilus is placed on the ventral surface, and eventually narrows to form the mouth or possibly (Criodrilus) closes at the position of the mouth. In Lumbricus trapezoides the oral opening is at first lined by hypoblast, and in Criodrilus is bounded anteriorly by three large peculiar epiblast cells, which arebelieved by Hatschek to assist in absorbing the albuminous fluid in which the eggs are suspended. These large cells are eventually covered by the normal epiblast cells and subsequently disappear. In both these types the hypoblast cells undergo, during their invagination, peculiar changes connected with their nutritive function.
In Euaxes (Kowalevsky) the segmentation is far more unequal than in the other types; a typical epibolic invagination takes place (fig. 140), and the blastopore closes completely along the ventral surface.
Section through ovum of EuaxesFig. 140. Transverse section through the ovum of Euaxes during an early stage of development.(After Kowalevsky.)ep.epiblast;ms.mesoblastic band;hy.hypoblast.
Fig. 140. Transverse section through the ovum of Euaxes during an early stage of development.(After Kowalevsky.)
ep.epiblast;ms.mesoblastic band;hy.hypoblast.
In all the oligochætous types, with the exception of Euaxes, where the blastopore closes completely, the blastopore becomes, or coincides with the mouth. In Serpula it is stated (Stossich), as we have seen, to coincide with the anus: a statement which receives confirmation from the similar statements of Willemoes-Suhm (No.358). It is necessary either to suppose a mistake on the part of Stossich, or that we have in Chætopods a case like that of Gasteropods in which a slit-like blastopore originally extending along the ventral surface may in some forms become reduced to a pore at the oral, or in other forms at the anal extremity.
So far only two germinal layers—the epiblast and the hypoblast—have been spoken of. Before the invagination of the hypoblast is completed the mesoblast makes its appearance in the form of two bands or streaks, extending longitudinally for the whole length of the embryo. These are usually spoken of as germinal streaks, but to avoid the ambiguity of this term they will be spoken of as mesoblastic bands.
Their origin and growth has been most fully studied by Kleinenberg (No.341) in Lum. trapezoides. They commence in this species shortly before the gastrula stage as two large cells on the surface of the blastoderm, which may be called mesoblasts. These cells lie one on each side of the median line at the hind end of the embryo. They soon travel inwards and become covered by the epiblast (fig. 141A,m´), while on their inner and anterior side a row of small cells appears (ms).These rows of cells form the commencement of the mesoblastic bands, and in the succeeding stages they extend one on each side of the body (fig. 141B,ms) till they reach the sides of the mouth. Their forward growth takes place mainly at the expense of the superjacent epiblast cells, but the two mesoblasts at their hinder extremities probably assist in their growth. Each mesoblastic band is at first composed of only a single row of cells, but soon becomes thicker, first of all in front, and becomes composed of two, three or more rows of cells abreast. From the above it is clear that the mesoblastic bands have, in L. trapezoides at any rate, in a large measure an epiblastic origin.
Three sections illustrating the development of Lumbricus trapezoidesFig. 141. Three sections illustrating the development of Lumbricus trapezoides.(After Kleinenberg.)ms.mesoblastic band;m´.mesoblast;al.archenteron;pp.body cavity.A. Horizontal and longitudinal section of an embryo which is dividing into two embryos at the gastrula stage. It shews the mesoblasts and the mesoblastic bands proceeding from them.B. Transverse section shewing the two widely separated mesoblastic bands.C. Transverse section at a later stage shewing the mesoblastic bands which have approached the ventral line and developed a body cavitypp.
Fig. 141. Three sections illustrating the development of Lumbricus trapezoides.(After Kleinenberg.)
ms.mesoblastic band;m´.mesoblast;al.archenteron;pp.body cavity.
A. Horizontal and longitudinal section of an embryo which is dividing into two embryos at the gastrula stage. It shews the mesoblasts and the mesoblastic bands proceeding from them.B. Transverse section shewing the two widely separated mesoblastic bands.C. Transverse section at a later stage shewing the mesoblastic bands which have approached the ventral line and developed a body cavitypp.
At first the two bands endin frontat the sides of the mouth, but subsequently their front ends grow dorsalwards at theexpense of the adjoining epiblast cells, and meet above the mouth, forming in this way a mesoblastic dorsal commissure.
The mesoblastic bands soon travel from the lateral position, which they at first occupy, towards the ventral surface. They do not however meet ventrally for some time, but form two bands, one on each side of the median ventral line (fig. 141C).
The usual accounts of the origin and growth of the bands differ somewhat from the above. By Kowalevsky (No.342) and Hatschek (No.339) they are believed to increase in Lumbricus rubellus and Criodrilus entirely at the expense of the mesoblasts. Kowalevsky moreover holds that in L. rubellus the original mesoblasts spring from the hypoblast. In some forms,e.g.Lumbricus agricola, the mesoblasts are not present.
In Euaxes the origin of the mesoblast bands is somewhat interesting as illustrating the relation of the Chætopod mesoblastic bands to the mesoblast of other forms. To render intelligible the origin of the mesoblast in this form, it is necessary to say a few words about the segmentation.
By a somewhat abnormal process of segmentation the ovum divides into four spheres, of which one is larger than the others, and occupies a position corresponding with the future hind end of the embryo. The three smaller spheres give riseon their dorsal sideby a kind of budding to small cells, which become the epiblast; and the epiblast is also partly formed from the hinder large cell in that this cell produces by budding a small cell, which again divides into two. The anterior of the two cells so formed divides still further and becomes incorporated in the epiblast; the posterior only divides into twowhich form the two mesoblasts. The remainder of the mesoblast is formed by further division of the three smaller of the primitive large spheres, and at first forms a continuous layer between the dorsal cap of epiblast and the four largest cells which, after giving rise to the epiblast and mesoblast, constitute the hypoblast. As the epiblast spreads over the hypoblast the mesoblastic sheet gives way in the middle, and the mesoblast remains as a ridge of cells at the edge of the epiblastic cup. It forms in fact a thickening of the lips of the blastopore. Behind the thickening is completed by the two mesoblasts. The appearance of the mesoblast in section is shewn infig. 140. As the epiblast accompanied by the mesoblast grows round the hypoblast, the blastopore assumes an oval form, and the mesoblast appears as two bands forming the sides of the oval. The epiblast travels over the hypoblast more rapidly than the mesoblast, so that when the blastopore becomes closed ventrally the mesoblastic bands are still some little way apart on the ventral side.
In Euaxes the mesoblast originates in a manner which is very similar to that in some of the Gasteropoda,e.g.Nassa, videp.234, and Vermes,e.g.Bonellia, etc. As mentioned in the chapter on theMollusca the origin of the mesoblast in Planorbis,p.227, is very similar to that in Lumbricus.
Hatschek has shewn that in Polygordius the mesoblast arises in fundamentally the same way as in the Oligochæta.
Besides the mesoblast which arises from the mesoblastic bands, there is evidence of the existence of further mesoblast in the larvæ of many Polychæta in the form of muscular fibres which traverse the space between the body wall and the wall of the enteric cavity prior to the formation of the permanent body cavity. These fibres have already been described in the embryo of Serpula, and are probably represented by stellate cells in the cephalic region (præ-oral lobe) of the Oligochæta. These cells are probably of the same nature as the amœboid cells in the larvæ of Echinodermata, some Mollusca and other types.
The Larval form.
True larval forms are not found in the Oligochæta where the development is abbreviated. They occur however in the majority of the marine Polychæta.
They present a great variety of characters with variously arranged ciliated bands. Most of these forms can be more or less satisfactorily derived from a larval form, like that of Serpula (fig. 139B) or Polygordius (fig. 142); and the constant recurrence of this form amongst the Chætopoda, combined with the fact that it presents many points of resemblance to the larval forms of many Rotifers, Molluscs, and Gephyreans, seems to point to its being a primitive ancestral form for all these groups.
The important characters of this larval form are (1) the division of the body into a large præ-oral lobe and a relatively small post-oral region containing the greater part of the alimentary tract; (2) the presence of a curved alimentary canal divided into stomodæum (œsophagus), stomach and intestine, and opening by a ventrally placed mouth, and an anus near the hind end of the body. To these may be added the frequent presence of (1) a ganglion at the apex of the præ-oral lobe, (2) a large cavity between the wall of the gut and the skin, which is the remnant of the segmentation cavity, and is usually traversed by muscular strands, of which one connecting the apex of the præ-oral lobe and the stomach or œsophagus is very commonly present (fig. 142).
The arrangement of the ciliated bands presents great variations,though in some instances it is constant through large groups. In Chætopods there is a widely distributed præ-oral ciliated band, which is similarly placed to the ring constantly found in the larvæ of Molluscs, Rotifers, etc. In many of these forms the band is practically double, the opening of the mouth being placed between its two component rings (videfig. 142). The best introduction to the study of the Chætopod larval forms will be the history of the changes of a typical larval form in becoming converted into the adult.