GENERAL ZOOLOGY 



Ambulacral 

 grooves 



Fig. 16.15. Class Crinoidea. 

 A feather star, Antedon rosaceus. 

 A, general appearance of the 

 animal, fi, oral view of the 

 central disk, showing the am- 

 bulacral grooves converging 

 on the mouth. The anus is 

 elevated on a conical struc- 

 ture. (Redrawn from H. C. 

 Chadwick, 1907, Liverpool 

 Marine Biological Committee 

 Memoirs, no. 15.) 



circulates and serves some of the functions of a blood-vascular system. As in 

 other echinoderms, however, the coelomic fluid is most important in this 

 respect. The reproductive system in holothurians is peculiar in that there is 

 only one gonad, the duct of which opens externally in the dorsal interambu- 

 lacral area between two of the tentacles. The animals are dioecious; gametes 

 are shed into the sea, fertilization is external, and as in other echinoderms 

 there is a bilateral, ciliated latval stage, called in this instance the 

 auricularia (Fig. 16.14). 



If its lack of a skeleton is disregarded, a sea cucumber is like a sea urchin 

 with its body elongated in the axis of radial symmetry, which extends from 

 mouth to anus. Correlated with this elongation are the anteroposterior and 

 dorsoventral differentiations and the appearance of a superficial bilateral 

 symmetry. 



506 



