132 THE GASTROPODA 
ecto-meres, and the latter proliferate very rapidly. In such forms 
as Patella, Planorbis, and Limaz, the blastula formed in this manner 
shgl 
Fia. 110. 
Development of the river-snail (Paludina 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; m, mouth; mes, 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 
