ECHINODERMATA 411 



E and E). The former is soon transformed into a five-lobed 



.^tructure, which grows around the stomodeeum of the larva, 



^^kid thus proves to be the water- vascular ring of the animal 



'^Fig. 195). Five outgrowths from it indicate the primary 



tentacles. 



The enterocoele has in the meantime grown out into a sac, 

 which has bent around under the intestine and then divided 

 into a right and a left peritoneal vesicle. These are sym- 

 metrically placed on the intestine (Fig. 195). They enlarge 

 and finally obliterate the cleavage cavity. Their cavity be- 

 comes the permanent body cavity, and their walls the peri- 

 toneum. Where their walls come together the mesentery of 

 the intestine arises. 



In the development of the entero-hydrocoele the Holothurioidea differ 

 from the forms previously considered, owing to the fact that the vaso- 

 peritoneal vesicle does not present a bilaterally symmetrical shape from 

 the beginning, this being expressed very late, not until the abstriction of 

 the enterocoele from the hydrocoele has taken place. The hydrocoele 

 in the Holothurioidea, in contrast to the other forms, separates very early 

 from the enterocoele. 



In the Crinoidea the conditions are quite peculiar, pro- 

 bably owing to the fact that the archenteron surrenders its 

 connection with the ectoderm at a very early period, and 

 lies as an isolated sac in the interior of the blastula. On 

 the third day this sac acquires an annular constriction 

 (Fig. 196 A), which later becomes considerably deeper, so 

 that there ai-ise two vesicles, which are connected by only 

 a narrow neck. The two vesicles may be distinguished as 

 anterior and posterior, for, corresponding to the subsequent 

 development of the embryo, an anterior and a posterior part 

 of the body can be distinguished even now. The anterior pole 

 is marked by the accumulation of numerous mesenchyma 

 cells, and the posterior by the position of the vesicular archen- 

 teron. The posterior of the two vesicles now changes its 

 shape by elongating in the transverse direction and then ac- 

 quiring a slight constriction at the middle (Fig. 196 B). 

 Two hollow processes grow out backward from the point of 

 junction of the two vesicles, and bend around the constricted 



