218 



KADIATES: ECHINODERMS. 



RADIATES. 



THESE animals are so constructed that their parts 

 radiate from a centre or central axis. In most of 

 them the radiation is very plain. They all live in the 

 water, and nearly all live in the sea, and are known 

 as Echinoderms, Jelly-Pishes, and Polyps. 



ECHINODERMS. 



The word Echinoderm means hedgehog-skin , and is 

 given to these animals because many of them have the 

 outside covered with spines ; thus reminding us of the 

 hedgehog of the fields, which was described on page 

 45. It is an interesting fact that in the Radiates the 

 parts are generally arranged according to what is called 

 a reigning number ; in Echinoderms this number is 

 generally five; that is, the parts of each kind are five, 

 or some multiple of five. 



HOLOTHURIANS. 

 The Holothurians, or Sea-Cucumbers, have no spines, 



Fig. 462. HolQthurian, or Sea-Cucumber. 



but are covered with a tough skin capable of great ex- 



