Eye of Loligo at two stagesFig. 125. Sections through the developing eye of Loligo at two stages.(After Bobretzky.)hl.inner segment of lens;vl.outer segment of lens;aanda´. epithelium lining the anterior optic chamber;gz.large epiblast cells of ciliary body;cc.small epiblast cells of ciliary body;ms.layer of mesoblast between the two epiblastic layers of the ciliary body;af.andif.fold of iris;rt.retina;rt´´.inner layer of retina;st.rods;aq.equatorial cartilage.
Fig. 125. Sections through the developing eye of Loligo at two stages.(After Bobretzky.)
hl.inner segment of lens;vl.outer segment of lens;aanda´. epithelium lining the anterior optic chamber;gz.large epiblast cells of ciliary body;cc.small epiblast cells of ciliary body;ms.layer of mesoblast between the two epiblastic layers of the ciliary body;af.andif.fold of iris;rt.retina;rt´´.inner layer of retina;st.rods;aq.equatorial cartilage.
Muscular system.The muscular system in all groups of Molluscs is derived entirely from the mesoblast.
The greater part of the system takes its origin from the somatic mesoblast. In almost all Gasteropod and Pteropod larvæ there is present a well-developed spindle muscle attaching the embryo to the shell. This muscle appears to be absent in the Cephalopoda.
Body cavity and vascular system.The body cavity in Gasteropods and Pteropods originates either by a definite splitting of the mesoblast, or by the appearance of intercellular spaces. It becomes divided into numerous sinuses which freely communicate with the vascular system.
Very different accounts have been given by different investigators of the development of the heart in the Gasteropoda and Pteropoda.
It would seem however in most cases to arise as a solid mass of mesoblast cells at the hind end of the pallial cavity, which subsequently becomes hollowed out and divided into an auricle and ventricle. Bobretzky’s careful observations have fully established this mode of development for Nassa.
In Pteropods the heart is formed (Fol) close to the anus, but slightly dorsal to it (fig. 108,h). The pericardium is formed from the mesoblast at a considerably later period than the heart.
A very different account of the formation of the heart is given by Bütschli for Paludina. He states that there appears an immense contractile sack on the left side of the body. This becomes subsequently reduced in size, and in the middle of it appears the heart, probably from a fold of its wall. The original sack would appear to give rise to the pericardium.
In connection with the vascular system mention may be made of certain contractile sinuses frequently found in the larvæ of Gasteropoda and Pteropoda. One of these is placed at the base of the foot, and the other on the dorsal surface within the mantle cavity immediately below the velum[114]. The completeness of the differentiation of these sinuses varies considerably; in some forms they are true sacks with definite walls, in other cases mere spaces traversed by muscular strands. They are found in the majority of marine Gasteropods, Heteropods and Pteropods. In Limax a large posteriorly placed pedal sinus is well developed, and there is also a sinus in the visceral sack. The rhythmical contraction of the yolk-sack of Cephalopods appears to be a phenomenon of the same nature as the contraction of the foot sinus of Limax.
In Calyptræa (Salensky) there is an enormous provisional cephalic dilatation within the velum which does not appear to be contractile. Similar though less marked cephalic vesicles are found in Fusus, Buccinum and most marine Gasteropods.
In Cephalopods the vascular system is formed by a series of independent (?) spaces originating in the mesoblast, the cells around which give rise to the walls of the vessels. The branchial hearts are formed at about the time at which the shell-gland becomes closed. The aortic heart (fig. 127,c) is formed of two independent halves which subsequently coalesce (Bobretzky).
The true body cavity arises as a space in the mesoblast subsequently to the formation of the main vascular trunks.
Renal organs.Amongst the Gasteropods and Pteropods there are present provisional renal organs, which may be of two kinds, and a permanent renal organ.
The provisional organs consist of either (1) an external paired mass of excretory cells or (2) an internal organ provided with a duct, which is not in all cases certainly known to open externally. The former structure is found especially in the marine Prosobranchiates (Nassa, etc.) where it has been fully studied by Bobretzky. It consists of a mass of cells on each side of the body, close to the base of the foot, and not far behind the velum. This mass grows very large, and below it may be seen a continuous layer of epiblast. The cells forming it fuse together, their nuclei disappear, and numerous vacuoles containing concretions arise in them. At a later stage all the vacuoles unite together and form a cavity filled with a brown granular mass.
The provisional internal renal organ is found in many pulmonate Gasteropods—Lymnæus, Planorbis, etc. It consists of a pairedV-shaped ciliated tube with a pedal and cephalic limb. The former has an external opening, but the termination of the latter is still in doubt.
It consists, according to Büschli’s description (No.244), in the freshwater Pulmonata (Lymnæus, Planorbis) of a round sack, close to the head, opening by an elongated and richly ciliated tube in the neighbourhood of the eye. From the sack a second shorter tube passes off towards the foot, which seems however to end blindly. The cells lining the sack contain concretions, and there is one especially large cell in the lumen of the sack attached on the side turned towards the eye. It coexists in Lymnæus with provisional renal organs of the type of those in marine Prosobranchiata.
A somewhat different description of the structure and development of this organ in Planorbis has recently been given by Rabl (No.268). It consists of aV-shaped tube on each side with both extremities opening into the body cavity. The one limb is directed towards the velar area, the other towards the foot. It is developed from the mesoblast cells of the anterior part of the mesoblastic band. The large mesoblast (p.227) of each side grows into two processes, the two limbs of the future organ. A lumen in the cell is continued into each limb, while continuations of the two limbs of theVare formed from the hollowing out of the central parts of the adjoining mesoblast cells.
In Limax embryos Gegenbaur found a pair of elongated provisional branched renal sacks, the walls of which contained concretions. These sacks are provided with anteriorly directed ducts opening on the dorsal side of the mouth. This organ isprobably of the same nature as the provisional renal organ in other Pulmonata.
Permanent renal organ.According to the most recent observer (Rabl,No.268), whose statements are supported by the sections figured, the permanent renal organ in Gasteropods is developed from a mass of mesoblast cells close to the end of the intestine. This is first carried somewhat to the left side, and then becomes elongated and hollow, and attaches itself to the epiblast on the left side of the anus (fig. 108,r). After the formation of the heart the inner end opens into the pericardium and becomes ciliated, the median part becomes glandular and concrements appear in its lining cells, and the terminal part forms the duct.
Previous observers have usually derived this organ from the epiblast; according to Rabl this is owing to their having studied too late a stage in the development.
In Cephalopoda the excretory sacks or organ of Bojanus are apparently differentiations of the mesoblast[115]. At an early stage part of their walls envelops the branchial veins. From this part of the wall the true glandular section of the organ would seem to be formed. The epithelium forming the inner wall of each sack is at an early age very columnar.
The development of the organ of Bojanus in Lamellibranchiata has been studied by Lankester. He finds that it develops as a paired invagination of the epiblast immediately ventral to the anus.
Generative glands.The generative glands in Mollusca would appear to be usually developed in the post-larval period, but our knowledge on this subject is extremely scanty.
In Pteropods Fol believes that he has proved that the hermaphrodite gland originates from two independent formations, one (the testicular) epiblastic in origin, and the other (the ovarian) hypoblastic.
These views of Fol do not appear to me nearly sufficiently substantiated to be at present accepted.
The generative glands in Cephalopoda appear to be simple differentiations of the mesoblast. They are at first very closelyconnected with the aortic heart (fig. 127,kd), but soon become completely separated from it.
Alimentary tract.The formation of the archenteron, and the relation of its opening to the permanent mouth and anus, has already been described and needs no further elucidation. It will be convenient to treat the subject of this section under three headings for each group—viz.(1) the mesenteron, (2) the stomodæum, and (3) the proctodæum.
The mesenteron.In the Gasteropoda and Pteropoda the mesenteron, as has already been mentioned, forms a simple sack, which may however, owing to the presence of food-yolk, be at first without a lumen. Of this sack an anterior portion gives rise to the stomach and liver, and a posterior to the intestine. This latter portion is the first to be distinctly differentiated as such, and forms a narrowish tube connecting the anterior dilatation with the anus. In the meantime the cells of a great part of the anterior portion of the mesenteron undergo peculiar changes. They enlarge, and in each of them a deposit of food material appears, which is often at any rate derived from the absorption of the albumen in which the embryo floats. The cells on the dorsal side, adjoining the œsophageal invagination, and the whole of the cells on the ventral side do not however undergo these changes. There thus arises an anterior and ventral region adjoining the œsophagus, which becomes completely enclosed by small cells and forms the true stomach. The part behind and dorsal to the stomach is lined by the large nutritive cells and forms the liver. It opens into the stomach at the junction of the latter with the intestine, which in the later stages becomes bent somewhat forwards and to the right. Still later the hepatic region becomes branched, the albuminous contents of its cells are replaced by a coloured secretion, and it becomes bodily converted into the liver. The stomach is usually richly ciliated.
The various modifications of the above type of development of the alimentary tract are to be regarded as due to the disturbing influence of food-yolk. Where primitively the hypoblast cells are very bulky, though invaginated in a normal way, the wall of the hepatic region becomes immensely swollen with food-yolk,e.g.Nautica. In other cases amongst certain Pteropods (Fol,No.249) where the hypoblast is still more bulky, part of the archenteric walls becomes converted into a bilobed sack openinginto the pyloric region, in the walls of which a large deposit of food material is stored, which gradually passes into the remainder of the alimentary tract and is there digested. The bilobed nutritive sack, as it is called by Fol, is eventually completely absorbed, though the liver in some, if not all cases, grows out as a fresh sack from its duct.
The formation of the permanent alimentary tract, when the hypoblast is so bulky that there is no true archenteric cavity, has been especially investigated by Bobretzky (No.242).
In the case of a species of Fusus the hypoblast, when enclosed by the epiblast, is composed of four cells only. The blastopore remains permanently open at the oral region, and around it the œsophagus grows in a wall-like fashion. The protoplasmic portions of the four hypoblast cells are turned towards the œsophageal opening, and from them are budded off small cells which are continuous at the blastopore with the epiblast of the œsophagus. These cells give rise posteriorly to the intestine and anteriorly to the sack, which becomes the stomach and liver. This sack always remains open towards the four primitive yolk cells. The cells of the posterior part of it become larger and larger and form the hepatic sack, which fills up the left and posterior part of the visceral sack, pushing the yolk cells to the right. The cells lining the hepatic sack become pyramidal in shape, and each of them is filled with a peculiar mass of albuminous material. The cells adjoining the opening of the œsophagus remain small, become ciliated, and form the stomach. They are not sharply separated off from the cells of the hepatic sack. The yolk cells remain distinct on the right side of the body during larval life, and their food material is gradually absorbed for the nutrition of the embryo.
A modification of the above mode of development, where the food material is still more bulky and the blastopore closed, is found in Nassa, and has already been described (videp.233).
The stomodæum.The stomodæum in most cases is formed as a simple epiblastic invagination which meets and opens into the mesenteron. When the blastopore remains permanently open at the oral region the stomodæum is formed as an epiblastic wall round its opening. In all cases the stomodæum gives rise to the mouth and œsophagus. At a subsequent period there are developed in the oral region of the stomodæum the radula in a special ventral pit, and the salivary glands—the latter as simple outgrowths.
The œsophagus is usually ciliated.
The proctodæum.Except where the blastopore remains as the permanent anus (Paludina) the proctodæum is always formed subsequently to the mouth. Its formation is usually preluded by the appearance of two projecting epiblast cells, but it isalways developed as a very shallow epiblastic invagination, which does not give rise to any part of the true intestine.
In the Cephalopods the alimentary tract is formed, as in other cephalophorous Mollusca, of three sections. (1) A stomodæum, formed by an epiblastic invagination, which gives rise to the mouth, œsophagus and salivary glands. (2) A proctodæum, which is an extremely small epiblastic invagination. (3) A mesenteron, lined by true hypoblast, which forms the main section of the alimentary tract,viz.the stomach, intestine, the liver, and ink sack[116].
Section through a Loligo ovumFig. 126. Longitudinal vertical section through a Loligo ovum when the mesenteric cavity is just commencing to be formed.(After Bobretzky.)gls.salivary gland;brd.sheath of radula;oe.œsophagus;ds.yolk-sack;chs.shell-gland; mt. mantle;pdh.mesenteron;x.epiblastic thickening between the folds of the funnel.
Fig. 126. Longitudinal vertical section through a Loligo ovum when the mesenteric cavity is just commencing to be formed.(After Bobretzky.)
gls.salivary gland;brd.sheath of radula;oe.œsophagus;ds.yolk-sack;chs.shell-gland; mt. mantle;pdh.mesenteron;x.epiblastic thickening between the folds of the funnel.
The mesenteron.The mesenteron is first visible from the surface as a small tubercle on the posterior side of the mantle between the rudiments of the two gills (fig. 111B,an). Within this, as was first shewn by Lankester, a cavity appears.
This cavity is as in Gasteropods open to the yolk-sack, and only separated from the yolk itself by the yolk membrane already spoken of. It is at first lined by indifferent cells of the lower layer of the blastoderm, which however soon become columnar and form a definite hypoblastic layer (fig. 126,pdh). Between the hypoblast and epiblast there is a very well marked layer of mesoblast. As the mesenteric cavity extends, its wallsmeet the epiblast, and at the point of contact of the two layers the epiblast becomes slightly pitted in. At this point the anus is formed at a considerably later period (fig. 127,an).
On the ventral side of the primitive mesenteron an outgrowth appears very early, which becomes the ink sack (fig. 127,bi).
Illustration: TitleFig. 127. Longitudinal section through an advanced embryo of Loligo.(After Bobretzky.)os.mouth;gls.salivary gland;brd.sheath of radula;ao.anterior aorta;ao1.posterior aorta;va.branch of posterior aorta to shell sack;ma.branch of posterior aorta to mantle;c.aortic heart;oe.œsophagus;mg.stomach;an.anus;bi.ink sack;kd.germinal tissue;eih.shell sack;vc.vena cava;g.vs.visceral ganglion;g.pd.pedal ganglion;ac.auditory sack;tr.funnel.
Fig. 127. Longitudinal section through an advanced embryo of Loligo.(After Bobretzky.)
os.mouth;gls.salivary gland;brd.sheath of radula;ao.anterior aorta;ao1.posterior aorta;va.branch of posterior aorta to shell sack;ma.branch of posterior aorta to mantle;c.aortic heart;oe.œsophagus;mg.stomach;an.anus;bi.ink sack;kd.germinal tissue;eih.shell sack;vc.vena cava;g.vs.visceral ganglion;g.pd.pedal ganglion;ac.auditory sack;tr.funnel.
The mesenteric cavity, still open to the yolk, gradually extends itself in a dorsal direction over the yolk-sack, but remains for some time completely open to it ventrally, and only separated from the actual yolk by the yolk membrane. There early grow out from the walls of the mesenteron a pair of hepatic diverticula.
As the mesenteric cavity extends it dilates at its distal extremity into a chamber destined to form the stomach (fig. 127,mg). At about this time the anus becomes perforated. Shortly afterwards the mesenteron meets and opens into the œsophagus at the dorsal extremity of the yolk-sack, but at the time when this takes place the hypoblast has extended round the entire cavity, and has shut it off from the yolk. The yolk membrane throughout the whole of this period is quite passive, and has no share in forming the walls of the alimentary tract.
The stomodæeum.The stomodæum appears as an epiblastic invagination at the anterior side of the blastoderm, before any trace of the mesenteron is present. It rapidly grows deeper, and, shortly after the mesenteric cavity becomes formed, an outgrowth arises from its wall adjoining the yolk-sack, which gives rise to the salivary glands (figs. 126and127,gls). Immediately behind the opening of the salivary glands there appears on its floor a swelling which becomes the odontophore, and behind this a pocket of the stomodæal wall forms the sheath of the radula (figs. 126and127,brd). Behind this again the œsophagus is continued dorsalwards as a very narrow tube, which eventually opens into the stomach (fig. 127).
The terminal portion of the rudiment of the salivary gland divides into two parts, each of which sends out numerous diverticula which constitute the permanent glands. The greater part of the original outgrowth remains as the unpaired duct of the two glands[117].
In the larva observed by Grenacher the anterior pair of salivary glands originated from independent lateral outgrowths of the floor of the mouth, close to the opening of the posterior salivary glands.
The yolk-sack of the Cephalopoda.The yolk, as has already been stated, becomes at an early period completely enclosed in a membrane formed of flattened cells, which constitutes a definite yolk-sack. It is, in the more typical forms of Cephalopoda, divided into an external and an internal section, of which the former is probably a special differentiation of the median part of the foot of other cephalophorous Mollusca (videp.272). At no period does the yolk-sack communicate with the alimentary tract. The two sections of the yolk-sack are at first not separated by a constriction. In the second half of embryonic life the condition of the yolk-sack undergoes considerable changes. The internal part grows greatly in size at the expense of the external, and the latter diminishes very rapidly and becomes constricted off from the internal part of the sack, with which it remains connected by a narrow vitelline duct.
The internal yolk-sack becomes divided into three sections: a dilated section in the head, a narrow section in the neck, and an enormously developed portion in the mantle region. It is the latter part which mainly grows at the expense of the external yolk-sack. It gives off at its dorsal end two lobes, which pass round and embrace the lower part of the œsophagus.The passage of the yolk from the external to the internal yolk-sack is probably largely due to the contractions of the former.
The external yolk-sack is not vascular, and probably the absorption of the yolk for the nutrition of the embryo can only take place in the internal yolk-sack. The most remarkable feature of the Cephalopod yolk-sack is the fact that it lies on the opposite side of the alimentary tract to the yolk cells, which form a rudimentary yolk-sack in such Gasteropoda as Nassa and Fusus. In these forms, the yolk-sack is at first dorsal, but subsequently is carried by the growth of the liver to the right side. In Cephalopoda on the contrary, the yolk-sack is placed on the ventral side of the body.
What is known of the development of the alimentary tract in the Polyplacophora has already been mentioned.
In the Lamellibranchiata (Lankester,No.239), the mesenteron early grows out into two lateral lobes which form the liver, while the part between them forms the stomach.
In Pisidium the intestine is formed from the original pedicle of invagination, which remains permanently attached to the epiblast. The stomodæum is formed by the usual epiblastic invagination, and becomes the mouth and œsophagus. The development of the crystalline rod and its sack do not appear to be known. In the adult the sack of the crystalline rod opens into a part of the alimentary tract which appears to belong to the mesenteron. Were however the development to shew them to be really derived from the stomodæum they might be interpreted as rudiments of the organ which constitutes the odontophore and its sack in cephalophorous Mollusca—an interpretation which would be of considerable phylogenetic interest.
Bibliography.
General.
(238)T. H. Huxley. “On the Morphol. of the Cephal. Mollusca.”Phil. Trans.1853.(239)E. R. Lankester. “On the developmental history of the Mollusca.”Phil. Trans.1875.(240)H. G. BronnandW. Keferstein.Die Klassen u. Ordnungen d. Thierreichs,Vol.III.1862‑1866.
Gasteropoda and Pteropoda.
(241)J. AlderandA. Hancock. “Devel. of Nudibr.”Ann. and Magaz. Nat. Hist.,Vol.XII.1843.(242)N. Bobretzky.“Studien über die embryonale Entwicklung d. Gasteropoden.”Archiv f. mikr. Anat.,Vol.XIII.(243)W. K. Brooks. “Preliminary Observations on the Development of Marine Gasteropods.”Chesapeake Zoological Laboratory, Session of 1878. Baltimore, 1879.(244)O. Bütschli.“Entwicklungsgeschichtliche Beiträge (Paludina vivipara).”Zeit. f. wiss. Zool.,Vol.XXIX.1877.(245)W. Carpenter. “On the devel. of the embr. of Purpura lapillus.”Trans. Micros. Soc.,2dseries,Vol.III.1855.(246)W. Carpenter. “On the devel. of the Purpura.”Ann. and Mag. of Nat. Hist.,2dseries,Vol.XX.1857.(247)E. Claparède.“Anatomie u. Entwickl. der Neritina fluviatilis.”Müller’sArchiv, 1857.(248)H. Eisig.“Beitr. z. Anat. u. Entwickl. der Geschlechtsorg. von Lymnæus.”Zeitschr. f. wiss. Zool.,Vol.XIX.1869.(249)H. Fol.“Sur le développement des Ptéropodes.”Archiv. de Zool. expérim. et générale,Vol.IV.1875.(250)H. Fol.“Sur le développement des Gastéropodes pulmonés.”Compt. rend.,1875,pp.523‑526.(251)H. Fol.“Sur le développement des Hétéropodes.”Archiv. de Zool. expérim. et générale,Vol.V.1876.(252)C. Gegenbaur.“Beit. z. Entwicklungsgesch. der Landgasteropoden.”Zeitschr. f. w. Zool.,Vol.III.1851.(253)C. Gegenbaur.Untersuch. üb. Pteropoden u. Heteropoden, Leipzig, 1855.(254)H. von Jhering.“Entwicklungsgeschichte von Helix.”Jenaische Zeitschrift,Vol.IX.1875.(255)W. KefersteinandE. Ehlers.“Beob. üb. d. Entwick. v. Æolis peregr.”Zool. Beitr.,1861.(256)J. KorenandD. C. Danielssen.“Bemärk. til Mollusk Udvikling.”Nyt Mag. f. Naturvidensk.,Vol.V.1847.Isis,p.202. 1848.(257)J. KorenandD. C. Danielssen.Bidrag til Pectinibr. Udvikl.Bergen, 1851 (supplement, 1852).Ann. and Mag. Nat. Hist., 1857.(258)A. Krohn.“Beobacht. aus d. Entwickl. der Pteropoden u. Heterop.”Müller’sArchiv, 1856 and 1857.(259)A. Krohn.Beitr. zur Entwickl. der Pteropoden u. Heteropoden.Leipzig, 1860.(260)H. de Lacaze-Duthiers.“Mém. sur l’anat. et l’embryog. des Vermets.”2epartie.Ann. sc. nat.,4esérie,T.XIII.1860.(261)P. Langerhans.“Zur Entwickl. der Gasterop. Opisthobr.”Zeitschr. f. w. Zool.,Vol.XXIII.1873.(262)E. R. Lankester. “On the development of the Pond-Snail.”Quart. J. of Micr. Scie.,Vol.XIV.1874.(263)E. R. Lankester. “On the coincidence of the blastopore and anus in Paludina vivipara.”Quart. J. of Micr. Scie.,Vol.XVI.1876.(264)F. Leydig.“Ueber Paludina vivipara.”Zeitschr. f. w. Zool.,Vol.II.1850.(265)J. Müller.Ueber Synapta dig. u. üb. d. Erzeug. v. Schnecken in Holoth., 1852.(266)J. Müller.“Bemerk. aus d. Entwickl. der Pteropoden.”Monatsber. Berl. Akad.,1857.(267)C. Rabl.“Die Ontogenie d. Süsswasser-Pulmonaten.”Jenaische Zeitschrift,Vol.IX.1875.(268)C. Rabl.“Ueb. d. Entwick. d. Tellerschnecke (Planorbis).”Morph. Jahrbuch,Vol.V.1879.(269)W. Salensky.“Beitr. zur Entwickl. d. Prosobr.”Zeitschr. f. w. Zool.,Vol.XXII.1872.(270)O. Schmidt.“Ueb. Entwick. von Limax agrestis.”Müller’sArchiv, 1851.(271)Max S. Schultze.“Ueber d. Entwick. des Tergipes lacinulatus.”Arch. f. Naturg., Jahrg.XV.1849.(272)E. Selenka.“Entwick. von Tergipes claviger.”Niederl. Arch. f Zool.,Vol.I.1871.(273)E. Selenka.“Die Anlage d. Keimbl. bei Purpura lapillus.”Niederl. Arch. f. Zool.,Vol.I.1872.(274)C. Semper.“Entwickl. der Ampullaria polita, etc.”Natuurk. Verhandl. Utrechts Genootsch., 1862.(275)An. Stecker.“Furchung u. Keimblatterbildung bei Calyptræa.”Morphol. Jahrbuch,Vol.II.1876.
(276)A. Stuart.“Ueb. d. Entwickl. einiger Opisthobr.”Zeitschr. f. w. Zool.,Vol.XV.1865.(277)N. A. Warneck.“Ueber d. Bild. u. Entwick. d. Embryos bei Gasterop.”Bullet. Soc. natural. de Moscou,T.XXIII.1850.
Cephalopoda.
(278)P. J. van Beneden.“Recherches sur l’Embryogénie des Sépioles.”Nouv. Mém. Acad. Roy. de Bruxelles,Vol.XIV.1841.(279)N. Bobretzky. Observation on the development of the Cephalopoda (Russian).Nachrichten d. kaiserlichen Gesell. d. Freunde der Naturwiss. Anthropolog. Ethnogr. bei d. Universität Moskau.(280)H. Grenacher.“Zur Entwicklungsgeschichte d. Cephalopoden.”Zeit. f. wiss. Zool.,Bd.XXIV.1874.(281)A. Kölliker.Entwicklungsgeschichte d. Cephalopoden.Zurich, 1844.(282)E. R. Lankester. “Observations on the development of the Cephalopoda.”Quart. J. of Micr. Science,Vol.XV.1875.(283)E. Metschnikoff.“Le développement des Sépioles.”Archiv d. Sc. phys. et nat.,Vol.XXX.Genève,1867.
Polyplacophora.
(284)A. Kowalevsky.“Ueb. d. Entwick. d. Chitonen.”Zoologischer Anzeiger,No.37. 1879.(285)S. L. Lovén.“Om utvecklingen hos slägtet Chiton.”Stockholm öfversigt,XII.1855. [VidealsoAnn. and Mag. of Nat. Hist.,Vol.XVII.1856, andArchiv f. Naturgeschichte, 1856.]
Scaphopoda.
(286)H. Lacaze-Duthiers.“Développement du Dentale.”Ann. d. Sci. Nat.,SeriesIV. Vol.VII.1857.
Lamellibranchiata.
(287)M. Braun.“Postembryonale Entwicklung d. Süsswasser-Muscheln.”Zoologischer Garten.(288)C. G. Carus.“Neue Untersuch. üb. d. Entwickl. unserer Flussmuschel.”Verh. Leop.-Car. Akad.,Vol.XVI.1832.(289)W. Flemming.“Studien in d. Entwicklungsgeschichte der Najaden.”Sitz. d. k. Akad. Wiss. Wien,Vol.LXXI.1875.(290)F. Leydig.“Ueber Cyclas Cornea.”Müller’sArchiv, 1855.(291)S. L. Lovén.“Bidrag til Känned. om Utweckl. af Moll. Acephala Lamellibr.”Vetensk. Akad. Handl.,1848. [VidealsoArch. f. Naturg., 1849.](292)C. Rabl.“Ueber d. Entwicklungsgeschichte d. Malermuschel.”Jenaische Zeitschrift,Vol.X.1876.(293)W. Salensky.“Bemerkungen über Haeckels Gastræa-Theorie (Ostrea).”Arch. f. Naturg.,1874.(294)O. Schmidt.“Ueb. d. Entwick. von Cyclas calyculata.”Müller’sArch., 1854.(295)O. Schmidt.“Zur Entwickl. der Najaden.”Wien, Sitzungsber. math.-nat. Cl.,Vol.XIX.1856.(296)P. Stepanoff.“Ueber die Geschlechtsorgane u. die Entwicklung von Cyclas.”Archiv f. Naturgeschichte,1865.(297)H. Lacaze-Duthiers.“Développement d. branchies d. Mollusques Acéphales.”An. Sc. Nat.,Ser.IV.Vol.V.1856.
[99]The classification of the Mollusca adopted in the present chapter is shewn in the subjoined table:I.ODONTOPHORA.1.Gasteropoda.a.Prosobranchiata.b.Opisthobranchiata.c.Pulmonata.d.Heteropoda.2.Pteropoda.a.Gymnosomata.b.Thecosomata.3.Cephalopoda.a.Tetrabranchiata.b.Dibranchiata.4.Polyplacophora.5.Scaphopoda.II.LAMELLIBRANCHIATA.a.Dimya.b.Monomya.[100]The reader is referred for the segmentation topp.98‑102, and to the special description of separate types.[101]Rabl (No.268) describes a blastopore of this form in Planorbis which closes at the mouth.[102]Rabl (No.268) has quite recently given a more detailed account than previous observers of the origin of the mesoblast in Planorbis. He finds that it originates from the posterior one of the four large cells which remain distinct throughout the segmentation. By the division of this cell two ‘mesoblasts’ are formed, one on each side of the middle line at the hinder end of the embryo. Each of these again divides into two, an anterior and a posterior. By the division of the mesoblasts there arise two linear rows of mesoblastic cells—the mesoblastic bands—which are directed forwards and divided transversely into two parts, an anterior continued from the front mesoblast, and a posterior from the hinder mesoblast.If Rabl’s account is correct, there is a striking similarity between the origin of the mesoblast in Mollusca and in Chætopoda. It appears to me very probable that the mesoblastic bands are formed (as in Lumbricus) not only from the products of the division of the mesoblasts, but also from cells budded off from one or both of the primary germinal layers.[103]In Octopus and Argonauta (Lankester) as soon as the blastoderm is completed the egg reverses its position in the egg-shell; the cleavage pole taking up a position nearest the stalk.[104]I do not know the relation of this axis of symmetry to the future embryo.[105]“Development of Pond-Snail.”Quart. J. of Micro. Science, 1874,pp.371‑374.[106]There is a striking similarity between the changes of the blastopore in Chiton and the formation of the neurenteric canal of Chordata; especially if Kowalevsky is correct in stating that the pedal nerves are developed from the ventral plate.[107]R. H. Peck, “Gills of Lamellibranch Mollusca.”Quart. J. of M. Science,Vol.XVII.1877.[108]The account of the remainder of the development till the larva becomes hatched is taken from Rabl,No.292.[109]In this description I follow Rabl’s nomenclature. According to his statements the ventral part of the body is the original animal pole—the dorsal the lower pole; the anterior end the mesoblastic side of the opening of invagination.[110]The position of the foot and gills in the larva represented inFig. 119B would be more normal if the convex and not the flatter side of the shell were the anterior. I have followed Rabl and Flemming in the determinations of the anterior and posterior end of the embryo, but failed to rear my larvæ up to a stage at which the presence of the heart or some other organ would definitely confirm their interpretation. I originally adopted myself the other view, and in case they are mistaken, the so-called velum would be a circumanal patch of cilia, while the position of the primitive mesoblast cells as well as of the byssus would better suit my view than that adopted in the text on the authority of the above observers.[111]The Anatomy of Invertebrated Animals,p.519.[112]Ussow states that they are independent.[113]For a fuller account of this subject the reader is referred to the chapter on ‘The Development of the Eye.’[114]Rabl holds that there is no contractile dorsal sinus, but that the appearance of contraction there is due to the contractions of the foot.[115]I conclude this from Bobretzky’s figures.[116]The following description applies specially to Loligo.[117]In Loligo only a single pair of salivary glands is present.
[99]The classification of the Mollusca adopted in the present chapter is shewn in the subjoined table:
I.ODONTOPHORA.
1.Gasteropoda.
a.Prosobranchiata.
b.Opisthobranchiata.
c.Pulmonata.
d.Heteropoda.
2.Pteropoda.
a.Gymnosomata.
b.Thecosomata.
3.Cephalopoda.
a.Tetrabranchiata.
b.Dibranchiata.
4.Polyplacophora.
5.Scaphopoda.
II.LAMELLIBRANCHIATA.
a.Dimya.
b.Monomya.
[100]The reader is referred for the segmentation topp.98‑102, and to the special description of separate types.
[101]Rabl (No.268) describes a blastopore of this form in Planorbis which closes at the mouth.
[102]Rabl (No.268) has quite recently given a more detailed account than previous observers of the origin of the mesoblast in Planorbis. He finds that it originates from the posterior one of the four large cells which remain distinct throughout the segmentation. By the division of this cell two ‘mesoblasts’ are formed, one on each side of the middle line at the hinder end of the embryo. Each of these again divides into two, an anterior and a posterior. By the division of the mesoblasts there arise two linear rows of mesoblastic cells—the mesoblastic bands—which are directed forwards and divided transversely into two parts, an anterior continued from the front mesoblast, and a posterior from the hinder mesoblast.
If Rabl’s account is correct, there is a striking similarity between the origin of the mesoblast in Mollusca and in Chætopoda. It appears to me very probable that the mesoblastic bands are formed (as in Lumbricus) not only from the products of the division of the mesoblasts, but also from cells budded off from one or both of the primary germinal layers.
[103]In Octopus and Argonauta (Lankester) as soon as the blastoderm is completed the egg reverses its position in the egg-shell; the cleavage pole taking up a position nearest the stalk.
[104]I do not know the relation of this axis of symmetry to the future embryo.
[105]“Development of Pond-Snail.”Quart. J. of Micro. Science, 1874,pp.371‑374.
[106]There is a striking similarity between the changes of the blastopore in Chiton and the formation of the neurenteric canal of Chordata; especially if Kowalevsky is correct in stating that the pedal nerves are developed from the ventral plate.
[107]R. H. Peck, “Gills of Lamellibranch Mollusca.”Quart. J. of M. Science,Vol.XVII.1877.
[108]The account of the remainder of the development till the larva becomes hatched is taken from Rabl,No.292.
[109]In this description I follow Rabl’s nomenclature. According to his statements the ventral part of the body is the original animal pole—the dorsal the lower pole; the anterior end the mesoblastic side of the opening of invagination.
[110]The position of the foot and gills in the larva represented inFig. 119B would be more normal if the convex and not the flatter side of the shell were the anterior. I have followed Rabl and Flemming in the determinations of the anterior and posterior end of the embryo, but failed to rear my larvæ up to a stage at which the presence of the heart or some other organ would definitely confirm their interpretation. I originally adopted myself the other view, and in case they are mistaken, the so-called velum would be a circumanal patch of cilia, while the position of the primitive mesoblast cells as well as of the byssus would better suit my view than that adopted in the text on the authority of the above observers.
[111]The Anatomy of Invertebrated Animals,p.519.
[112]Ussow states that they are independent.
[113]For a fuller account of this subject the reader is referred to the chapter on ‘The Development of the Eye.’
[114]Rabl holds that there is no contractile dorsal sinus, but that the appearance of contraction there is due to the contractions of the foot.
[115]I conclude this from Bobretzky’s figures.
[116]The following description applies specially to Loligo.
[117]In Loligo only a single pair of salivary glands is present.
Entoprocta.
The development of the larvæ 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 single-layered 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[119]pole become invaginatedin the normal manner (fig. 128A), the blastopore becomes narrowed to a slit with an anteroposterior 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. 128B,me), one on each side of the middle line. These cells give rise to the mesoblast. On the completion of the invagination the mesoblasts become covered by the epiblast (fig. 128C,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.