PHYLUM ECHINODERMATA 



169 



is sometimes absent ; in Asterias vulgaris it is minute, 

 almost closed. The spines and pedicellariaj differ in their 

 form and arrangement in different kinds of starfish, as also, 

 though in a less degree, the tube-feet. The starfishes con- 

 stitute one of the five classes of living Echinodermata, the 

 class Aster oidea. 



2. THE OPHIUROIDEA 



The Brittle-stars bear many resemblances to the true star- 

 fishes, but have a number of special features of sufficient 



Fig. 93.— Ophioglypha lacertosa. A. outline, of the natural size; B, central disc, 

 dorsal view; C, the disc, ventral view showing the mouth and genital fissures. 

 (From Nicholson and Lydekker's PaltBontology \) 



importance to justify their being regarded as constituting a 

 separate class, which is termed the Ophiuroidea. Like 



