PHYLUM MOLLUSCA 



753 



ectoderm which pushes the blastopore inwards and forms the 

 rudiment ot* the stomodseum. The originally solid mass of 

 endoderm develops a lumen, and its cells become arranged to 

 form the enteric epithelium. From the posterior end, where 

 the mesoderm cells are situated, proceed two very regularly 

 formed mesoderm-bands (Fig. 645, B). On the dorsal surface 

 the shell-gland has already appeared as a pit lined by elongated 

 ectoderm cells ; on the surface of this appears the embryonic 

 shell. The rudiment of the foot (Fig. 645, A) arises at a re- 

 markably early stage as two protuberances lying on the ventral 

 side of the posterior end of the larva at the sides of the blastopore ; 

 these coalesce to form the median foot. 



too, r,4.">. — a and B, Trochophores of Patella at different stages. In A are to be seen the 

 circular blastopore and the two foot-elevations ; in B the blastopore is drawn out, at the sides 

 of it are the two mesoderm bands. (From Korschelt and Ucider, after Patten.) 



The larva (Fig. 646) has now assumed the trochophore form. The 

 pre-oral part is large and convex, with an apical plate on which is 

 borne a bunch of long cilia, and near it two small ciliated elevations, 

 each consisting of a single cell. The pre-oral part of the larva 

 then becomes much flattened, and the apical plate (ap. pi) increases 

 in size and importance. At the posterior end is a bunch of cilia 

 which are borne on two special large cells, the anal cells (an. e). The 

 embryonic shell becomes saucer-shaped. A slight ridge in the 

 neighbourhood of the shell represents the border of the mantle. 

 The mid-gut (mesent) has become considerably widened : a 

 diverticulum from it is recognisable, and this afterwards opens 

 on the exterior to form the anus. A diverticulum of the fore- 

 gut (rati) at the same time forms the rudiment of the radular 

 sac. The statolith-sacs appear as depressions of the ectoderm at 



