XII 



PHYLUM MOLLUSCA 



733 



On the dorsal surface the shell-gland has already appeared as a 

 pit lined by elongated ectoderm cells ; on the surface of this appears 



B 



FIG. 655. 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 Heider, after Patten.) 



the embryonic shell. The rudiment of the foot (Fig. 655, A) 

 arises at a remarkably 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. 656) 

 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 be- 

 comes much flattened, 

 and the apical plate (ap. 

 pi) increases in size and 

 importance. At the pos- 

 terior end is a bunch of 



VOL. I 



me sent 



cen.c 



FIG. 656. Later trochophore of larva of Patella in 

 longitudinal section, on. c. anal cells with cilia ; ap. pi. 

 apical plate ; /. foot ; mes. mesoderm cells ; mesetit. 

 mesenteron ; mo. mouth; rad. rudiment of radula-sac ; 

 sh. shell. (From Korschelt and Heider, after Patten.) 



3 A 



