RADIATES. 



173 



The radiates constitute the greater part of the group 

 of beings known as zoophytes ; they have been divided 

 into several classes, the more important of which are 

 the echinodermata, the acalepha, the corals, and the 

 sponges. 



We will examine a few types adapted to give us gen- 

 eral ideas concerning these classes. 



To the echinodermata belong the sea-urchins and the 



star-fish. 



FIG. 101. 



SEA-UKCHIN. 



The sea-urchins have a sensibly rounded form, the 

 body being protected by a calcareous envelope that is 

 studded with thorns. These creatures are found in all 

 seas, and form a large number of species. The common 

 sea-urchin is about as large as a fairly large apple j it 



