THE PHYLA HEMICHORDATA AND ECHINODERMATA 



375 



Figure 1 9.1 5. The basket star, Gorgonocephalus, showing the branched arms. (Cour- 

 tesy of the American Museum of Natural History.) 



1 71 . Class Ophiuroidea, the Brittle Stars 



Brittle stars have slender rays attached to a circular disc (Fig. 19.5 A). 

 Each ray is composed in part of a row of large cylindrical skeletal pieces 

 called vertebrae joined together with short but powerful muscles. Each 

 ray as a whole is very supple, and brittle stars move by pushing and 

 pulling on surrounding objects, "slithering" like a snake (Fig. 19.7 B). 

 The tube feet are poorly developed and lack suckers. They function 

 primarily as tactile sense organs. The delicate ciliated epidermis that 

 covers most of the skeleton in other echinoderms is replaced in this class 

 by a tough cuticle. Pieces of the rays are easily broken off, but are easily 

 legenerated. 



Some ophiuroids, such as Gorgonocephalus (Fig. 19.15) of the west 

 coast, are called basket stars because the arms branch and intertwine 

 repeatedly. One wonders how basket stars are able to keep track of all 

 the branches as they clamber through vegetation, and indeed they have 

 been observed to leave behind pieces that are hopelessly entangled. 



Most ophiuroids feed on debris and mud. Some capture prey with 

 their prehensile rays and bring it to the mouth. The mouth opens into 

 a simple saclike stomach where food is digested and adsorbed. Indi- 

 gestible remains must be eliminated through the mouth, for no other 

 digestive organs are present. 



172. Relationships among Echinoderm Classes 



The most primitive echinoderms of which we have any record are 

 believed to be members of the extinct class Heterostelea (Fig. 19.16), 



