ECHINODERMATA 



395 



only the lower interests us for the present. Its blind end 

 enlarges a little, and comes into contact with the ventral 

 wall of the larva (Fig. 178). The corresponding part 

 of the wall sinks in, forming a cup-like depression (Fig. 

 179), the intestine fuses with it, and an opening leading 

 into the intestine now breaks through at this point : the 

 mouth-opening of the larva. The mouth-opening is there- 

 fore a new formation. The intestine opens to the outside 

 at the posterior end by means of the blastopore ; the 



Fig. 176. 



Figs. 176 and 177.— Larvas of Synapta digitata, showing the formation of the dor- 

 sal pore (P) and the vaso-peritoneal vesicle (after Sblenka). Bl, blastopore. 



gastrula mouth consequently has become the anus of the 

 larva. 



"With these changes the larva has also undergone a certain 

 differentiation in extei'nal shape. Its bilateral symmetry is 

 ah-eady expressed. The mouth and anus mark definite 

 regions of the body. The former lies on the ventral surface, 

 the latter at the posterior end of the larva. Moreover, as we 

 will mention in anticipation of the sequel, the ventral surface 



