Branch Echinodermata 45 



and covered with a shell made up of closely united plates 

 of a calcareous nature, to which stiff spines are attached. 

 Sea-urchins live in rocky holes and crevices along the 

 , borders of the ocean, where they feed upon molluscs and 

 seaweeds. Their mode of locomotion is similar to that 

 of the starfish, although the movement is much slower. 



IV. — Holothuroidea. The strange sea-cucumbers or sea- 

 slugs belong to this class. In these the body is elon- 

 gated and covered with a thickened leathery wall. 

 Around the mouth is a ring of tentacles. These slug- 

 gish creatures live in the ocean, especially in tropical 

 regions. 



V. — Crinoidea. The members of this class are commonly 



known as feather-stars or sea-lilies. They are com- 

 paratively few in number, and for the most part being 

 fixed to the bottom of the sea, they are seldom found. 

 The star-shaped body is usually attached to a stalk. 

 In former geologic times crinoids were very abundant ; 

 the formation of vast limestone areas is attributed to 

 the gradual accumulation of their remains. 



