Echmoderm LarvcB ivith Two Water- Vascular Systems. 333 



larval stomach meets a central depression of the amniotic floor and in this 

 way the adult mouth and oesophagus are formed. 



It will be obvious, therefore, that in the formation of the Echinus-rudiment 



8 



no less than three distinct tissues are involved. These are (1) the hydroccele, 

 (2) the amniotic floor, and (3) the wall of the left posterior coelom. One of 

 the main objects of the experiments described in this paper was to determine, 



