132 



THE GASTROPODA 



ecto-meres, and the latter proliferate very rapidly. In such forms 

 as Patella, Planorbis, and Limax, the blastula formed in this manner 



Fio. 110. 



Development of the river-snail (I'aludiiui vivijnrd), in which the 

 5 formed independently of the blastopore, the latter persisting as tin 



mouth and stomodaeum 



are formed independently of the blastopore, the latter persisting as the anus, ae, archenteron, 

 or endodermic cavity; an, anus; II, blastopore ; d.c, directive or polar corpuscle; d.v, velar 

 area or cephalic dome ; /, foot ; in, mouth ; meg, rudiments of the mesoderin ; 71.?, pedicle of 

 invagination, the future rectum ; sk.gl, the shell-gland ; s.m, site of the as yet unformed mouth ; 

 IT, velum. A, Qastrula phase (optical section). IS, the Gastrula has become a Trochosphere 

 by the development of the ciliated velar ring (optical section). C, side view of the Trochosphere 

 with commencing formation of the foot 1), further advanced Trochosphere (optical section). 

 K, the Trochosphere passing into the Veliger stage ; dorsal view showing the formation of the 

 shell-gland. F, side view of the .same, showing foot, shell-gland, velum, mouth, and aims. 

 ( After Lankeater.) 



contains a large blastocoel between the nricromereu and macromeres, 

 but in other Gastropods this cavity is much reduced. In some 



