CLASS OF THE ECHINODERMATA. 527 



dermata have a circulatory apparatus sufficiently developed ; 

 and of all the zoophytes are those whose organization is most 

 complex and most perfect. They live in the sea, and when 

 young undergo some remarkable metamorphoses. The echino- 



Fig. 553. Encrinus ; Sea Lily. 



dermata form three principal groups the holothuria (Fig. 

 554), the echinus (Fig. 552), and the asterias or sea star (Fig. 

 159). Some species of this last family attach themselves by 

 a sort of stalk. Such are, or rather were, the encrinidae 



