Phylum Echinodermata and Related Groups 553 



have remarkable powers of regeneration. When attacked they often 

 throw out their internal organs including the respiratory tree and 

 digestive tract and simply grow another set. 



They feed chiefly on small organisms found in the mud and sand 

 where they live. These are entangled on the mucus of the tentacles. 



Economically these are not important forms, although in some 

 countries they are collected, dried, and sold in markets as "beche-de-mer" 

 or "trepang." 



THE CLASS CRINOIDEA 



The attached flowerlike crinoids, the sea Hlies (Fig. 183,a and k), 

 seem totally unlike the other echinoderms. Careful examination, how- 

 ever, shows that they have many characteristics in common. 







Fig. 183. — Various echinoderms. a, Metacriniis, a stalked crinoid; b, Thyone, 

 a sea cucumber; c, Echinarachnius, a sand dollar; d, Ophiura, a brittle star; e, 

 Leptosynapta, a sea cucumber; /, Arbacea, a sea urchin; g, the slate-pencil sea 

 urchin; h, Oreaster, a starfish; i, Gorgonocephalus, the basket star; ;, Lovenia, the 

 heart urchin; k, Antedon, the feather star. 



An attached form has a body disc which forms a cup-shaped calyx 

 to which are attached five, flexible, much-branched arms. The branches 

 are the pinnules. The calyx is attached to the long jointed stem which 

 extends downward from the aboral surface. It is usually attached to 

 the sea bottom by cirri, rootlike outgrowths. The mouth and anus are 

 both on the oral or upper surface of the disc ; there is no madreporite and 

 no pedicellariae. Along each arm is an anibulacral groove containing 

 cilia and modified tube feet. Usually the gonads are located in the pin- 

 nules of the arms. The eggs are shed directly into the water in some 



