158 



MOLLUSCA 



The embryonic cells continue to divide, and form an oval 

 vesicle containing liquid (fig. 149); within this, at one pole, 

 is seen the mass of in vagina ted cells (fig. 150, fit/). These 

 invaginated cells are the arch-enteron ; they proliferate and 

 give off branching cells, which apply themselves (fig. 1 50, 

 C) to the inner face of the vesicle, thus forming the meso- 



Fm. 152. Diagram of embryo of Pisidium in the same stage as E in fig. 151. 

 m, mouth ; /, foot ; ph, pharynx ; gs, met-enteron ; pi, rectal peduncle or 

 pedicle of invagination ; shs t shell-gland. (From Lankester.) 



blast or ccelomic outgrowths. The outer single layer of 

 cells which constitutes the surface of the vesicle (fig. 

 147) is the ectoderm or epiblast or cleric cell-layer. The 

 little mass of hypoblast or 

 enteric cell -mass now en- 

 larges, but remains con- 

 nected with the cicatrix of 

 the blastopore or orifice of 

 invagination by a stalk, the 

 rectal peduncle (fig. 151, A, 

 rp). The enteron itself be- 

 comes bilobed and is joined 

 by a new invagination, that 

 of the mouth and stomo- 

 dreum, ph. Fig. 151, B 

 shows the origin of the 

 mouth o, being a deeper 



view of the Same specimen Fio. 153. Diagram of embryo of Pisidium, 



,, ... * , . , in same stasje as F in fig. 151 (after Lan- 



m the Same position wllicn kester). m, month ; x, anus ; /, foot ; br, 



ic /li-TnTn in firr 1^1 A branchial filaments ; ma, margin of the 



ng. J -J i , .rt.. mantle . skirt . BI organ of Bojanus (ne- 



The mesoblast multiplies phridium). The unshaded area gives 

 its cells, which become the position of the shell-valve. 



partly muscular and partly skeleto-trophic. Centro-dor- 

 sally now appears the embryonic shell-gland (fig. 151, 

 C, sh). The pharynx or stomodoeum is still small, the 

 foot not yet prominent. A later stage is seen in fig. 

 152, where the pharynx is widely open and the foot pro- 

 minent. No ciliated velum or pne-oral (cephalic) lobe 

 ever develops. The shell-gland disappears, the mantle- 

 skirt is raised as a ridge (fig. 151, E, mn), the paired 

 shell-valves are secreted, the anus opens by a proctodoeal 

 ingrowth into the rectal peduncle, and the rudiments of 

 the gills (In-) and of the nephridia (B) appear (figs. 151, 

 F, and 153, dorsal and lateral views of same stage), and 

 thus the chief organs and general form of the adult are 



acquired. Later changes, not drawn here, consist in the 

 growth of the shell-valves over the whole area of the 

 mantle-flaps, and in the multiplication of the gill-fila- 

 ments and their consolidation to form gill -plates. It 

 is important to note that the gill-filaments are formed 

 one by one posteriorly. The labial tentacles are formed 

 late. In the allied genus Cyclas, a byssus gland is formed 

 in the foot and subsequently disappears, but no such gland 

 occurs in Pisidium. The nerve-ganglia and the otocysts 

 probably form from thickenings of the epiblast, but detailed 

 observation on this and other points of histogenesis in the 

 Lamellibranchia is still wanting. 



List of Memoirs, c., referred to by numbers in the preceding article. (1) G. 

 Cuvier, Memoires pour servir a I'histoire et a I'auatomie des Mollusques, Paris, 

 1816. (2) J. Poli, Tcstacea utriusque Sieitix, eorumque historia et anatome, 

 tabulis aeneis 49 illuitrata, vols. i.-iii., fol., Parma, 1791-1795 and 1820-1827. 



(3) St delle Chiaje, Memorie sulla storia e notomia degli animali senza verttbre 

 ilel regno di Napoli, Naples, 1823-1829: new edit, with 172 plates, fol., 1S43. 



(4) J. Vaughan Thompson, Zoological Researches, Cork, 1830 ; memoir iv., "On 

 the Cirripedes or Barnacles, demonstrating their deceptive character." (5) A. 

 Kowalewsky, " Entwickelungsgeschichte der einfachen Ascidien," in Mem. de 

 VAcad. dcs Sciences de St Petersbourg, I860, and "Entwickelungsgeschichte 

 des Amphioxits lanceolalus," ibid., ISO". (6) J. Vaughan Thompson, Zoological 

 Researches, Cork, 1830 ; memoir v., " Polyzoa, a new animal discovered as 

 an inhabitant of some Zoophytes." (7) C. G. Ehrenberg, Die Korallenthiere dcs 

 Ruthen Meeres, Berlin, 1834 (Abhand. d. k. Akad. d. Wisscnschaften in Berlin, 

 1S32). (8) H. Milne-Edwards, Recherches anatomiques physiologist's et zoolo- 

 giques sur ks Polypiers de France, Paris, 1841-1844. (9) W. H. Caldwell, "On 

 the development of Phoronis," Proc. Roy. Soc., 1SS2. (10) Richard Owen, Memoir 

 on the Pearly Nautilus, London, 1832. (11) T. H. Huxley, " On the morphology 

 of the cephalous Mollusca," Phil. Trans., 1853. (12) E. Ray Lankester, "Con- 

 tributions to the developmental history of the Mollusca," Phil. Trans., 1S75. 

 (13) E. Ray Lankester, "Notes on Embryology and Classification," Quart. 

 Journ. Microsc. Sc., 1877. (14) J. Carriere, " Das Wassorgefiiss-System d. Lamelli- 

 branchiaten u. Gastropoden," Zoolog. Anzeiger, 1881, No. 90. (15) E. Ray 

 Lankester, "Development of the Pond-Snail," Quart. Journ. Microsc. Sc., 1874, 

 and " Shell-gland of Cyclas and Planula of Limna;us," ibid., 1876. (16) R. 

 Horst, "Development of the European Oyster," Quart. Journ. Microsc. Sc., 

 1S82, p. 341. (17) E. Ray Lankester, "Coincidence of the blastopore and 

 anus in Paludina," Quart. Journ. Microsc. Sc., 1876. (18) Id., "Zoological 

 Observations made at Naples," Annals and Mag. Kat. Hist., February, 1S73. 

 (19) W. K. Brooks, "Development of the American Oyster," Report of the 

 Commissioners of Fisheries of Maryland, I860. (20) Henri Milne-Edwards, 

 Papers in the Annales des Sciences Katurelles, 1S41-1S60. (21) H. de Lacaze 

 Duthiers, Papers in the Annales des Sciences Naturdhs, e.g., " Anomia " (1854), 

 " Mytilus"(1856), " Dentalium " (1856, 1857), " Purpura " (1859), " Haliotis "(1859), 

 " Vermetua " (1860). (22) A. Kolliker, EnturickelungsgeschicMt der Cephalopoden, 

 Zurich, 1844. (23) C. Gegenbaur, Untersuchungen uber Pteropoden und Hetero- 

 poden, Leipsic, 1855. (24) J. W. Spengel, " Die Geruchsorgane und das Nerven- 

 system der Mollusken," Zeitschr. f. wiss. Zool, 1881. (26) A. A. W. Hubrecht, 

 "On Proneomenia Shuteri nov. gen. et sp., Tvith remarks upon the anatomy 

 and histology of the Amphineura," Niederldndisches Archivfiir Zooloyie, supple- 

 ment volume, 1SS1. (26) Adam Sedgwick, "On certain points in the anatomy 

 of Chiton," Proc. Roy. Soc. Land., 1881. (27) E. Ray Lankester, "On some 

 undescribed points in the anatomy of the Limpet," Annals and Mag. Nat. 

 History, 1867; J. T. Cunningham, "The Renal Organs of Patella," Quart. Journ. 

 Microsc. Sc., 1883. (28) P. Fraisse, " Ueber Molluskenaugen mit embryonaleiu 

 Typus," Zeitschr. f. wiss. Zool., 1881. (29) L. v. Gran", " Ueber Rhodope Vcranil, 

 Roll.," Morpholog. Jahrb., vol. viii. (30) H. Simroth, " Das Fussnervensystem 

 der Paludina vivipara," Zeitschr. f. wiss. Zool., 1881. (31) E. Ray Lankester, 

 "A contribution to the knowledge of Hemoglobin," Proc. Xoy. Soc. Land., 

 1873. (32) H. de Lacaze Duthiers, " Du systtme nerveux des Mollusques 

 Gasteropodes Pulmones aquatiques et d'un nouvel organe d'innervation," Arch, 

 de Zoologie erpcrimentale, vol. i. (33) C. Semper, Animal Life (for eye of 

 Onchidium, p. 371), International Scientific series, 1881. (34) Same as number 



A". Akad. r. ll'et. Katurk., Amsterdam, 1856. (37) E. Kay Lankester and A. G. 

 Bourne, "On the existence of Spengel's olfactory organ and of paired genital 

 ducts in the Pearly Nautilus," Quart. Journ. Microsc. Sc., 1883. (38) J. W. 

 Vigelius, "Ueber das Excretions-System der Cephalopodeu," Niederldndisches 

 Archiv fiir Zoologie. bd. v., 1880. (39) Albany Hancock, "On the nervous 

 system of Ommaslrephes todarus," Annals and Mag. Nat. }Iist., 1852. (40) 

 J. D. Macdonald, "On the anatomy of Nautilus umbilicatns," Phil. Trans, oj 

 Hoy. Soc. Lond., 1855. (41) V. Hensen, " Ufber das Augeeiniger Cephalopoden," 

 Zeitschr. f. wiss. Zool, 1865. (42) A. d'Orbigny, Mollusques rimnts et fossiles, 



Smelt," Proc. Zool. Soc. Loml, 1883. (45) Same as 35. (46) F. M. Balfour, 

 I'omimrativc Embryology, vols. i. and ii., London, 1881-1882. (47) H. Grics- 

 baeh, " Ueber das Gefass-System und die Wasseraufnahme bei den N.i.jaden und 

 Mytilidcn," Zeitschr. f. wiss. Znal., lxS3. (48) Same as 14. (4a) Same as 13. 

 (50) R. Holman Peek, "The structure of the Lamellibranchiate gill," Quart. 

 Journ. of Microsc. Sc., 1870. (51) K. Mitsukuri, "Structure and significance 

 of some aberrant forms of Lamellibranehiate gills," Quart. Journ. Microsc. Sc., 

 issi. (52) K. Langer, "Das Gefass-System der Teichmussd," Deal;, kais. 

 Akail. d. ll'isscnsch., Vienna, 1805-1850. (53) J. Penrose, in "Report of the 

 OiimmitU'.' on the Zoological Station of Naples," British Assoc. Report, 1882. 

 (54) P. P. C. Hock, " Les orgaues de la generation de 1'huitrc," Journ. de la Soc. 

 Ncerlutulaise de Zool, 1883. (E. R. L.) 



