ECHINODERMATA. 



103 



OllDER II. ASTEROIDEA. 



As the structure of the Sea-urchins may be taken as embodying 

 the most important anatomical peculiarities of the Echinodermata, 

 and as this has been described at some length, it will not be neces- 

 sary to do more than briefly indicate the more important character- 

 istics of the remaining orders. In the present order are included 

 all the true Star-fishes ; the Sand-stars and Brittle-stars being now ^ 

 regarded as a distinct group. The body in all the Asteroidea is 



Fig. 61. — The Common Star-fish (Uraster rubens), natural size, viewed from above. 



more or less obviously star-shaped (fig. 64), consisting of a central 

 disc surrounded by five or more lobes or arms, which radiate from 

 the body, are hollow, and contain prolongations from the stomach. 

 The body is not enclosed in an immovable box or test, as in the 

 Sea-urchins, but the integument is of a leathery nature, and is 

 richly furnished with calcareous plates, tubercles, and spines. The 

 true Star-fishes are distinguished from the nearly allied Brittle-stars 

 (Ophiuroidea) by the fact that the arms or " rays " are direct pro- 

 longations of the body, that they contain prolongations of the 



