132 



THE GASTROPODA 



ectomeres, and the latter proliferate very rapidly. In such forms 

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



E 



shgl 



FIG. 110. 



Development of the river-snail (Pahulina vivipara), in which the mouth and stomodaeum 

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

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

 area or cephalic dome ; /, foot ; in, mouth ; TOPS, rudiments of the mesoderm ; p.i, pedicle of 

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

 vr, velum. A, Gastrula phase (optical section). B, 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. D, further advanced Trochosphere (optical section). 

 E, 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 anus. 

 (After Lankester.) 



contains a large blastocoel between the micromeres and macromeres, 

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



