124 



GASTROPODA. 



going on. Since cilia appear as early as the blastula-stage (Fig. 49), 

 the embryo is very soon able to move about freely and thus becomes 

 a larva. In this way, Patella resembles a Lamellibranch, but such 

 early locomotion is not common among the Gastropods, most of the 

 larvae hatching at a much later stage. The ingrowth of entoderm 



FIGS. 49 and 50. Embryos of Patella at the blastula-stage and at the commencement 

 and completion of gastrulation (after PATTEN), bl, blastopore ; em, mesentomere ; 

 ent, entoderm ; mes, mesoderm ; sd, shell-gland ; w, ciliated ring. 



and the differentiation of the mesoderm take place, as already de- 

 scribed (pp. 114 and 121), from the thickened vegetative pole of the 

 blastula (Figs. 49 and 50). The blastopore lies at the vegetative 

 pole which at the same time corresponds to the posterior end of the 

 larva. The principal axis of the larva, at this stage, passes through 



