406 



EMBRYOLOGY 



into a five-lobed, rosette-like structure, in which the sub- 

 sequent shape of the water-vascular system, with its five 

 chief stems, is already expressed. The two chief vesicles, 

 which remain after the abstriction of the hydroccele, undergo 

 a metamorphosis similar to that which we shall describe 

 farther on for Asterina gibbosa. We only add at present 

 that they represent the fundament of the body cavity, the 

 enterocoele. 



It can be seen from the above description that in the 

 establishment of the vaso-peritoneal vesicles a bilateral 

 symmetry is expressed, which, however, is again deranged 



..-.I'"/,, 



\m/. 



B 



.^ 



^^ 



Vrr 



m 



^^y: 



y 



'"'^J^Ai^di^i^i^'^' 



Fig. 190. — A and B, two starfish larvae at the time of the development of the 

 entero-hydi'ocoele (afcer Metschnikoff). D, intestine ; Fp, vaso-peritoneal vesicle, 

 the dorsal pore of which can be recognized in B ; p, peritoneal vesicle (right-hand 

 enterocoele). 



by the development of only one hydroccele. A more exact 

 bilateral symmetry, extending to the formation of the 

 hydroccele, appears, on the other hand, to exist during these 

 stages in the 



Ophluroidea. — In this group also, according to the 

 statements of Metschnikoff (No. 37), two vaso-peritoneal 

 vesicles are constricted off from the intestine, but each of 

 them is said to divide into an enterocoelic and a hydrocoelic 

 vesicle. Ordinarily, however, only the anterior left one of 

 the two hydrocoeles develops further, whereas the right 



