PHYLUM ECHINODERMA 



177 



are ambulatory while the remaining ones lack the sucking disc 

 and seem to have chiefly respiratory and tactile functions. A 

 pair of branched, filamentous gills occurs on the margin of the 

 peristome opposite each interambulacral area of the sea urchin. 

 The siphon, as already mentioned, is also thought to aid in 

 respiration. 



The nervous system is fundamentally the same type as that 

 described for the Asteroidea, comprising a nerve ring from which 

 five primary branches are given off. In addition to pedicellariae, 

 there are small organs known as sphaeridia scattered over the 



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Fig. 



\Inksh'ne 



Anus nedvm 



86 B. — Anatomy of keyhole urchin, 

 Mellita pentapora. {After Coe). 



Fig. 86 A. — Dorsal view of 

 young echinopluteus of Echi- 

 nus esculentus. {Redrawn 

 from MacBride, courtesy of 

 Macmillan Co.). 



body surface of many species. These are thought to function 

 as organs of equilibrium. Specially modified tube-feet on the 

 peristome are associated with the sense of taste. 



Arbacia, Strongylocentrotus, and Toxopneustes are genera 

 of the common sea urchins. The tropical genus Clypeaster 

 includes some of the largest urchins. This, with the sand dollar 

 (Echinarachnius) and the keyhole urchin (Mellita, Fig. 86 B) 

 represents the order Clypeastroidea. Members of the order 

 Spatangoidea, of which Spatangus is an example, are usually more 

 or less heart-shaped. There are several groups of echinoids 

 which are known only from fossil remains. 



Class Holothuroidea 



The sea cucumbers are elongated echinoderns lacking a 

 definite skeleton, with a mouth at one extremity surrounded by 



