298 



INVEETEBEATA 



CHAP. 



By this time twenty-four hours have elapsed, and the embryo has 

 escaped from the egg-membrane and entered on its free-swimming 

 life as a Trochophore larva. In this larva the blastopore becomes 

 shifted forwards and then completely closed, but the stomodaeum 



ttr 



Fig. 233. — Three diagrams of sagittal sections of the early larva of Patella coerulea in order 

 to illustrate the shift of the blastopore and the formation of the stomodaeum. 

 (After Patten.) 



A, stage where the blastopore is at tlie vegetative pole of the egg. B, stage where both telotroch 

 and blastopore have been shifted forward owing to the growth of the dorsal region. C, stage where 

 blastopore is shifted farther forward hy growth of ventral region, the telotroch remaining stationary, 

 and in which the stomodaeum has originated. The arrow indicates the regions of the embryo which 

 are undergoing rapid growth, ap, apical plate ; end, endodermal cells ; p.tr, prototroch ; stma, stomo- 

 daeum ; t,tr, telotroch. 



arises as an invagination of the ectoderm at the spot where the last 

 trace of the blastopore disappeared. The shifting is due to the 

 greater relative growth of the dorsal ectoderm behind the proto- 

 troch (Fig. 233). The endodermic cells divide and form a sac which 

 is the stomach ; and from the hinder end of this the intestine 



