Fig. 20 

 B, diagrams of the segmentation of a gastropod, mac, macromeres ; niic, 

 micromeres. C, optical section of a blastula at the end of segmentation. 

 lie, blastoccele. D, E, two stages of gastrulation. bl, blastopore; mes, 

 mesomere. F, ventral view, and G, side view of the trochosphere of a 

 streptoneurous gastropod. /<l, blastopore ; F, foot ; mes, mesoblast bands ; 

 shg, shell gland. H and /, further stages showing the development of the 

 mantle m, and the larval shell sk, and the approximation of the mouth m, to 

 the anus, an. K, stage immediately after the larval torsion, showing the 

 altered relations of mouth and anus, t, cephalic tentacles ; ot, otolith ; F, foot. 

 L-N. final stages of development, showing the atrophy of the velum, the growth 

 of the permanent shell, and the development of the foot and head (the larval 

 shell is shaded with dti^lfesiSf*}**''- .;h4K'«*'l' ''"<==)■ ""^' mantle cavity ; 

 op, operculum, (Figs. G-^after Boutan.) 



