364 



THE ANIMAL KINGDOM 



between the proboscis and trunk, and cannot be considered to be the 

 central nervous system. Except for the collar nerve the entire system is 

 at the body surface and is covered only with epidermis. 



165. Classification of the Phylum Echinodermata 



Living echinoderms are divided into five classes (Fig. 19.5): (1) 

 Crinoidea, the sea lilies and feathers stars, (2) Holothuroidea, the sea 

 cucumbers, (3) Echinoidea, the sea urchins and sand dollars, (4) Aster- 

 oidea, the starfish, and (5) Ophiuroidea, the brittle stars and basket 



Figure 19.5. The five living classes of the Echinodermata. A, Ophiuroidea, brittle 

 stars. B, Asteroidea, starfishes. C, Echinoidea, sea urchins. D, Holothuroidea, sea cucum- 

 bers. E, Crinoidea, sea lilies. (C after Hunter and Hunter, others after Hyman.) 



Stars. In addition a number of extinct echinoderms have been iden- 

 tified that are placed in some five additional classes. Most echino- 

 derms are large and have skeletons, and many of the species are or have 

 been abundant. This phylum has a rich fossil record, probably the best 

 known of any phylum, that reaches back to the early part of the Paleo- 

 zoic Era. The number of known extinct species greatly outnumbers the 

 number of known living species. 



The five living classes are so different in their structural features 

 that space does not permit an adequate description of each one. The 

 general aspects of the classes will be given following a detailed descrip- 

 tion of a member of the Asteroidea. 



166. Asterias forbesif a Typical Five-rayed Starfish 



Asterias lives on rocky or shell-covered bottoms where it preys 

 extensively on shellfish. The common species of the east coast, A. forbesi 



