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 abstrictiou of the hydrocele, 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-peintoneal vesicles a bilateral 

 symmetry is expressed, which, however, is again deranged 



41% 

 c/.. 



- D 



^mmm 



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

 entero-hydrocosle (after Mktschnikoff). D, intestine ; Vp, 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 hydrocoele. A more exact 

 bilateral symmetry, extending to the formation of the 

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

 stages in the 



Ophiuroidea. — 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 enteroccelic and a hydrocoelic 

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

 the two hydrocoeles develops further, whereas the right 



