ECHINODERMATA 



409 



The processes which we have followed in the development 

 of the enterocoele and hydrocoele of Asterina bear a close 

 resemblance to those w^hich give rise to the formation of the 

 vaso-peritoneal system of the 



I^m Echinoidea. — As in Asterina, the blind end of the arch- 

 ^kteron is transformed into the fundament of the vaso- 

 peritoneal vesicle (Selenka, No. 53). Two outpocketings 

 are developed from it, the two being constricted off in 

 common from the intestine (Fig. 193 A to C). It is not until 

 later that they separate into a right and left vesicle ; the 

 former constitutes a part of the enterocoele, but the latter 

 represents not only the other part of the enterocoele, but also 

 the hydrocoele. Accordingly by another constriction the 

 left vesicle is divided into two, and in this manner gives rise 



Fig. 193.—^ to C, longitudinal sections of the archenteron of Echinus miliaris, 

 showing the development of the vaso-peritoneal vesicle (pfter Sblenka). 



to the left enterocoelic sac and the hydrocoele. The same 

 process is said by Metschnikoff to take place in the right 

 vesicle, so that a hydrocoele is- formed on the right side also, 

 in a manner similar to that described for the Asteroidea and 

 Ophiuroidea. The right hydrocoele is said subsequently to 

 degenerate. These statements are noteworthy only for the 

 reason that they may indicate that the water- vascular 

 system is traceable to an organ of paired origin. 



A mode of origin of the hydro-enterocoele still more 

 modified than that in the forms hitherto considered is 

 exhibited in the 



Holothurioidea, although it can be referred to the same 

 plan. We have already seen in Synopta (comp. Figs. 176 to 

 179, pp. 395, 396) that a part of the archenteron detaches 



