SURVEY OF INVERTEBRATES 



85 



The nervous system is essentially a nerve ring encircling the mouth, 

 with a branch extending into each arm, but there is no central, 

 dominating nerve mass that is comparable to a brain, and only 



Fig. 47. — A, Sea Urchin, Asthenosoma, oral view; B, Sea Cucumber, Cucu- 

 maria; C, Sea Lily, Metacrinus. (From Thomson, Ludvig, and Carpenter.) 



the simplest sense organs, the tube feet acting largely in the latter 

 capacity. The functions of a circulatory and respiratory system 

 are carried on by the coelomic fluid — mostly sea-water in which 

 amoeboid cells float — that fills the large body cavity, or coelom, 

 circulates through the dermal branchiae, and bathes all the organs, 

 including the male or female reproductive organs. 



