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 invaginated cells (fig. 150, hy). These 

 invaginated cells are the arch-enteron ; they proliferate and 

 give off branching cells, which apply themselves (fig. 150, 

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



Fio. 152.— Diagram of embryo of Pisidium in the same stage as B in flg. 151. 

 m, month ; /, foot ; ph, pharynx ; js, met-enteron ; jrf, rectal peduncle or 

 pedicle of inTaginatlcn ; sht, shell-gland. (From Lankester.) 



blast or coelomic outgrowths. The outer single layer of 

 cells which constitutes the surface of the vesicle (fig. 

 147) is the ectoderm or epiblast or deric ceU-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- 

 dseum, ph. Fig. 151, '&'^' 

 shows the origin of the 

 mouth 0, being a deeper 



view of the same specimen Fra. 153.— Diagram of embryo of Pisidinm, 



,, '!_• -L* -u in same stage as F in fig. 151 (after Lan- 



m the same position wnicn kester). m, mouth ; x, anus ; / foot ; Ir, 



\<! Ara-am in Ha I'll A branchial filaments ; mu, margin of the 



IS drawn in ng. IJI, -"■■ mantle-skirt ;.B, organ of Bojanus (ne- 



The mesoblast multiplies phrldium). The unshaded area gives 

 its cells, which become t^e 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 stomodseum 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 prae-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 proctodseal 

 ingrowth into the rectal peduncle, and the rudiments of 

 the gills (6r) and of the nephridia (B) appear (figs. 151, 

 r, 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 

 LameUibranchia is still wanting. 



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

 Cuvier, M4moires pour servir a I'histoire et a Vanattmie des Mollusgues^ Paris, 

 1816. (2) J. Poli, Testacea utriusq-ue Siciliiey eoritmque historia, et anatonie^ 

 tabulis aeneis 49 illustrata^ vols, i.-iii., fol., Parma, 1791-1795 and 1826-1827. 



(3) St delle GhiEye, Memorie sulla storia e twiomia degli animali senza vertebre 

 del regno di Napoli, Naples, 1823-1829 ; new edit, with 172 plates, fol., 1843. 



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

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

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

 VAcad. des Sciences de St Mtersionrg, 1866, and "Entwickelungsgeschichte 

 des AmpMoams lanceolatus" ibid.^ 1867. (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 des 

 Eothen Meeres, Berlin, 1834 (^Abhand. a. Jc. Akad. d. Wissenschaften in Berlin, 

 1832). (8) H. Milne-Edwards, Recherches anaiomiques physiologiques et zoolo- 

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

 the development of Phoronis," Proc. Boy. Soo., 1882. (10) Bichard Owen, Mennair 

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

 of the cephalous MoUusca," Phil. Trans., 1863. (12) B. Bay Lankester, "Con- 

 tributions to the developmental history of the Mollusca," Phil. Trans., 1875. 

 (13) B. Bay Lankester, "Notes on Embryology and Classification," Quart. 

 Joum. Mierosc. Sc, 1877. (14) J. Carri6re,"Das Wassergelass-Systemd. Lamelli- 

 branchiaten u. Gastropoden," Zoolog. Anzeiger, 1881, No. 90. (Ifi) E. Bay 

 Lankester, " Development of tlie Pond-Snail," Quart. Joum. Mierosc. Sc, 1874, 

 and " Shell-gland of Cyclas and Planula of Limnsens," ibid., 1876. (16) E. 

 Horst, "Development of the European Oyster," Quart. Joum, Mierosc. Sc, 

 1882, p. 341. (17) B. Bay Lankester, "Coincidence of the blastopore and 

 anus in Paludina," Quart. Joum. Mierosc Sc, 1876. (18) Id., "Zoological 

 Observations made at Naples," Annals and Mag. Nat. Hist., February, 1873. 

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

 ConmissioTiers of Fisheries of Maryland, 1880. (20) Henri Milne-Edwards, 

 Papers in the Annates des Sciences Naturelles, 1841-1860. (21) H. de Lacaze 

 Duthiers, Papers in the .<lnfia2£s des Sciences Naturelles, e.g., "Anomia" (1854), 

 " Mytilus " (1856), " Dentalium " (1856, 1867), " Purpura " (1859), " Haliotis " (1859), 

 " Vermetus " (I860). (22) A. Kolliker, Entwickelungsgeschichte der Cephdlopoden, 

 Zurich, 1844. (23) C. Gegenbaur, Untersuchungen ilber Pteropoden v/nd Hetero- 

 poden, Leipsic, 1856. (24) J. W. Spengel, " Die Geruchsorgane und das Nerven- 

 system der MoUusken," Zeitschr. f. wiss. Zool., 1881. (25) A. A. W. Hubrecht, 

 "On Proneamxnia Shuteri nov. gen. et sp., with remarks upon the anatomy 

 and histology of the Amphineura," Niederldndisches ArchivfUr Zoologie, supple- 

 ment volume, 1881. (26) Adam Sedgwick, "On certain points in the anatomy 

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

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

 History, 1867 ; J. T. Cunningham, "The Benal Organs of Patella," Quart. Joum. 

 Mierosc Sc, 1883. (28) P. Fraisse, " Ueber MoUuskenaugen mit embryonalem 

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

 K61I.," Morpholog. Jdkrb., vol. viii. (30) H. Simroth, "Das Fussnervensystem 

 der Paludina vivipara," Zeitschr. f. wiss. Zool., 1881. (31) B. Bay Lankester, 

 "A contribution to the knowledge of Hsemoglobin," Proc. Boy. Soc. Lond., 

 1873. (32) H. de Lacaze Duthiers, "Du systeme nerveux des MoUusques 

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

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

 Onchidium, p. 871), International Scientific series, 1881. (34) Same as number 

 18. (3S) E. Bay Lankester, " Observations on the development of the Cephalo- 

 poda," Quart. Joum. Mierosc Sc, 1876. (36) J. van der Hoeven, "Bydrage 

 tot de outleedkundige kennis aangaaende Nautilus pompilius," Verhandl. d. 

 K. Akad. v. Wet. Naturk., Amsterdam, 1856. (37) E. Bay Lankester and A. G. 

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

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

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

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

 system of Ommasirephes todarus," Annals and Mag, Nat. Hist., 1852. (40) 

 J. D. Macdonald, "On the anatomy of Nautilus umAilicatiis," PhU. Trans, of 

 Roy. Soc Lond,, 1855. (41) V. Hensen, " Ueber das Auge einiger Cephalopoden," 

 Zeitschr, f, wiss, Zool,, 1865. (42) A. d'Orbigny, MoUusques vivants et fbssiles, 

 t. i. (Cephalopodes aeetabulif^res), Paris, 1845 (with 36 plates). (43) Bobretzky, 

 "On the development of the Cephalopoda," Trans, of Soc, of Friends of Nat, 

 Hist, of Moscow, vol. xxiv. (Russian). (44) T. H. Huxley, "Oviducts of the 

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

 Comparaiive Embryology, vols. i. and 11., London, 1881-1882. (47) H. Gries- 

 bach, "Ueber das Gefass-System und die Wasseraufnahme bei den Najaden und 

 Mytillden," Zeitschr, f, wiss, Zool., 1883. (48) Same as 14. (49) Same as 13. 

 (50) E. Holman Peck, "TThe structure of the Lamellibranchiate gill," Quart, 

 Joum. of Mierosc. Sc, 1876. (51) K. Mitsukuri, "Structure and significance 

 of some aberrant forms of Lamellibranchiate gills," Quart. Joum. Mierosc Sc, 

 1881. (52) K. Langer, "Das Gefass-System der Teichmussel," Denk, kais, 

 Akad. d. Wissensch., Vienna, 1855-1856. (53) J. Penrose, in "Ileport of the 

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

 (54) P. P. 0. Hoek, " Les orgaues de la gen^ation de rhultre," Joum. de la Soc 

 Nea-landalse de Zool, 1883. (B. B. L.) 



