PHYLOGENY OF THE PELECYPODA. 



297 



wliifh bi'ood the young, as Uiiio and Pisidium, the vehim is hig-lily reduced or wanting*. 

 Fi'oni Ilorst, we may gather the eonehision that the metemhryo (gastrnhi) stage in the 

 European oyster, fig. 17, is not a pure gastrula, but has ah'eady acquired by concentra- 

 tion of development, some of the characters which would normally appear later. 



Professor Brooks says that the primitive blastojiore closes over, becomes the dorsal as- 

 pect of the emljrj'o and gives rise to the shell; the permanent mouth and anus forming 

 on the opposite side of the embryo and independent of the blastopore. This differs so 

 essentially from the observations of Dr. Horst in the European oyster and from those of 

 other investigators except Rabl, in the development of various groups of mollusca, that 

 we prefer to follow Dr. Horst's observations. Professor Brooks himself says (9), p. 70, 

 that the whole evidence furnished by comparative anatomy and embiyology tends to 

 show that the mouth, anus and shell-gland can be homologized perfectly in all the classes 

 of true molluscs and that they are not only homologous with each other, bilt must be 

 perfectly homologous also with similar structiu'es in the ancestral form of which the 

 molluscs are modifications. It is evident that these sevei'al organs would not be "■' per- 

 fectly homologous " if in one case the shell-gland arose from the closed blastopore and in 

 another from the opposite pole of the body, and the same would be ti'ueof the derivation 

 of the mouth and anus. 



Dr. Horst in considering the development of the preconchylian gland^ sa3^s that when 

 the embryo is viewed from the side the glandular depression is seen as in our fig. 18, sZ;,and 

 in fig. 17 in optical section. During the further development, our figs. 19 and 21, a small 



eK 



i..(Z 



FIG. 19. 



Fig. 20. 



Fig. 23. 



DEVEI.OI'MENT OF OSTKEA EDULIS (AFTEIl HOUST). 



Fig. 19. Embryo same stage as fig. 18, optical section ; 

 me, mesoderm ; d, primary gastrula iiivagiiiatiou ; sk, shell- 

 glancl. 



Fig. 20. Embryo a day older, front view, with the prim- 

 itive mouth opening. 



Fig. 21. The same embryo, optical section. 

 Fig. 22. Embryo a day older, with a wreatli of vibra- 

 tile cilia, a stomachic cavity and the beginning of a shell, s. 



invagination is formed bordered by walls of high cylindrical cells with a narrow opening. 

 Considering fig. 19, lIoi-st says that the gastrula cavity has deepened, and behind the 

 mouth are cells, probably mesodermic. The blastopore continues to be very distinct, and 

 as far as he could ascertain, it did not disappear but remained and was transformed in- 

 to the mouth. 



' The preconchylian gland was discovered by Lankester 

 (44) in 1871, and has been figured by him (44) in Loligo, 

 Aplysla, Pleurobranchidium, Limnasus and Pisidium. It 

 has also been observed and figured by Fol in a Heteropod 

 (see Balfour) ; by Rabl in Planorbis (see Clans) ; by 

 ButscUli, iu Paludina (see Claus) ; by Bobretsky, in 



Nassa (see Balfour); by Patten, in Patella; by Brooks 

 in a pnlmonate Gasteropod ; by Hatschek, in Teredo ; by 

 Horst in Ostrea and in other genera by various inves- 

 tigators. This array of genera confirms the opinion of 

 Lankester that it is typical of all developing molluscs. 



