ECHINODERMATA 407 



one in most cases degenerates, only occasionally giving 

 rise to a right-hand water-vascular rosette with a dorsal 

 pore (MtJLLER, Metschnikoff). In exceptional cases in 

 the Asteroidea also, there is said to be formed, in addition 

 to the left-hand one, a right-hand hydrocoelic vesicle, which, 

 quite like the other, develops into a five-rayed water- vascular 

 rosette, provided with a dorsal pore. In that case, a hydro- 

 coele and an enterocoele would therefore be present on both 

 sides, and the symmetry would be complete. 



The development of enterocoele and hydrocoele does not 

 take place in all Asteroidea in the way described, for 

 the double character in the fundament of the entero- 

 hydrocoele may not be so prominent, owing to the fact 

 that the vesicles of the two sides are no longer constricted 

 off from the archenteron separately, a condition which is 



^^^onstricted off from it (after Goette). The dorsal pore is already formed ; r and I, 

 right and left sacs of the vaso-peritoneal vesicle; A, anus; M, region of the 

 mouth, which does not develop until later. B, vaso-peritoneal vesicle of Asttrina 

 gihhosa, with its right and left sacs (r and I), on the latter the fundament of the 

 hydrocoele (H) (after Ludwig). 



important for the reason that it is a transition to the corre- 

 sponding process in the Echinoidea. Goette observed in 

 larvae of Asteracanthion glacialis that the process usually 

 takes place in the manner described, but that during the 

 abstriction from the archenteron the vesicles may in this 

 same form remain united with each other (Fig. 191 A). 

 Now, according to Ludwig, this latter condition is the 

 common one in Asterina. The vaso-peritoneal vesicle 

 appears in the form of two lateral outfoldings at the blind 

 end of the archenteron (Fig. 192 A). The two outfoldings 

 grow toward the posterior pole of the larva (Fig. 192 ^), 



