702 



ZOOLOGY 



SECT. 



elongation (Fig. 608, A) and eventually becomes parti} 7 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. COS, 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 



B 



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 sides 

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



shell. The rudiment of the foot (Fig. 60S, 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 

 prse-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 (me sent) has become considerably widened : a 



