154 



PHYLUM ECHINODERMATA. 



bule disappears and the tentacles and mouth become freely 

 exposed. We thus reach the so-called Cystid stage of develop- 

 ment (Fig. 110). 



The larvae of Cri- 

 noids, then, become 

 attached by the 

 ventral side of the 

 anterior end, and, 

 as was shown by 

 Bury, the stalk of 

 the adult is a de- 

 velopment of the 

 preoral lobe. They 

 therefore resemble 

 the larvae of Aste- 

 roids in the fact 

 that attachment 

 takes place by the 

 preoral lobe, but 

 differ entirely from 

 them in the relation 

 which the preoral 

 lobes bear to the 

 arrangement of the 

 organs in the adult ; 

 for whereas in Aste- 

 roids the preoral 

 lobe is encircled by 

 the water- vascular 

 ring, and its with- 

 ered vestige springs 

 from the oral sur- 

 face of the adult 

 disc (Fig. 105), in 

 Crinoids it is quite 



FIG. 110. Cystid larva of Antedon (after Thomson). 



free of the circum- 



oral vessel and arises from the apical or aboral surface of the 

 adult. 



Our knowledge of the development of the coelom is mainly 

 due to Bury, who in his memoir on the development of 



