702 



ZOOLOGY 



SECT. 



elongation (Fig. 608, A) and eventually becomes partly closed up, 

 the closure taking place from behind forwards ; the most anterior 

 part remains open to form the mouth, or, perhaps more correctly,, 

 there is in the position of the anterior part a sinking-in of the 

 ectoderm, which pushes the blastopore inwards and forms the 

 rudiment of the stomodreum. 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 streaks (Fig. 608, 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 



FIG. 60S. A and B, Trochospheres 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 side& 

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



shell. The rudiment of the foot (Fig. 608, A} appears 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. 



The larva (Fig. 609) has now assumed the trochosphere form. The 

 prae-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 prse-oral part of the larva 

 then becomes much flattened, and the apical plate (ap.pl.) 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.c.\ The 

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

 neighbourhood of the shell represents the border of the mantle. 

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



