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 E in fig. 151. 

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

 pedicle of invaginatiou ; shs, 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 deric cell-layer. The 

 little mass of hypoblast or 

 enteric cell-mass now en- 

 larges, but remains con- 

 nected with the cicatrix of \ V >n 

 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, B 

 shows the origin of the 

 mouth o, being a deeper 



view Of the Same specimen Fio. 153.-Diagram of embryo of Pisidinm, 



. . r , . , in same stage as F in iig. 151 (after Lan- 



m the same position Which kester). m, mouth ; x, anus ; /, foot ; br, 



rtra-nrti in fio- 1^1 A branchial filaments ; mn, margin of the 



III iig. iux, A. man tie-skirt ; B, organ of Bojanus (ne- 



Tll6 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 stomodeeum 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 prie-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 (br) 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'anatomie des Mollusques, Paris, 

 1816. (2) J. Poll, Testacea utriusque Sicilies, eorumque historia et anatome, 

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



(3) St delle Chiaje, Memarie sulla storia e notomia degli animali senza vertebre 

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



(4) J. Vaughan Thompson, Zoologiml Kesearch.es, Cork, 1830 ; memoir iv., "On 

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

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

 I'Acad. des Sciences de St Pettrsbourg, 1860, and " Entwickelungsgeschichte 

 des Amphiaxus lanceolatus," iliil., 1807. (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 

 Rothen Meeres, Berlin, 1834 (Abhand. d. k. Akad. d. Wissenschaften in Berlin, 

 1S32). (8) H. Milne-Edwards, Recherches anatomiques physiologiques et zpolo- 



branchiaten u. Gastropoderi," Zoolog. Anzeiger, 1881, No. 90. (15) E. Kay 

 Lankester, " Development of the Pond-Snail," Quart. Journ. Mic 



" 



. 



icrosc. Sc., 1874, 



, 



and "Shell-gland of Cyclas and Planula of Limnaeus," ibid., 1876. 

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



(16) R. 



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

 anus in Paludina," Quart. Journ. Microsc. Sc., 1870. (18) Id., "Zoological 

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

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

 Commissioners of Fisheries of Maryland, 1860. (20) Henri Milne-Edwards, 



Vermetus " (1800). (22) A. Kolliker, Entwickelungsgeschichle der Cepltalopoden, 

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

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

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



jf 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 embryonalem 

 Typus," Zeitschr. f. mss. Zool, 1881. (29) L. v. Graff, " Ueber Rhodope Veranit, 

 Koll.," Morpholog. Jahrb., vol. viii. (30) H. Simroth, " Das Fussnervcnsystem 

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

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

 1873. (32) H. de Lacaze Duthicrs, "Du systeme nerveux des Mollusques 

 Gasteropodes Pulmones aquatiques et d'un nouvelorgane d'innervation," Anh. 

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

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

 18. (35) E. Ray Lankester, " Observations on the development of the Cephalo- 

 poda," Quart. Journ. Microsc. Sc., 1875. (36) J. van der Hoeven, " Bijdrage 

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

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

 Bourne, "On the existence of Spongers olfactory organ and of paired genital 

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

 Vi^elius "Ueber das Excretions-System der Cephalopoden," Niederldndisches 

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

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

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

 Roy. Soc. Lond., 1S55. (41) V. Hensen, " Ueber das Auge einiger Cephalopoden," 

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

 t. i. (Cephalopodes acetabuliferes), 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). () T. H. Huxley, " Oviducts of the 

 Smelt," Proc. Zool. Soc. Lond., 1SS3. (45) Same as 35. (46) F. M. Balfour, 

 Comparative Embryology, vols. i. and ii., London, 1881-1882. (47) H. Gries- 

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

 Mytiliden," Zeitschr. f. wiss. Zool., 1SS3. (48) Same as 14. (49) Same as 13. 



Akad. d. Wiss'ensch., Vienna, 1805-1866. (53) J. Penrpse, in "Report of the 

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



(54) P. P. C. Hoek, "Les orgaiics de la generation de 1'huitre," Journ. de la Soc. 

 Neerlandaise ae Zool, 1883. (E. R. L.) 



