PHYLUM ECHINODERMATA 



205 



seek their food at the surface of the mud and secure small living plants 

 and animals by means of their tentacles. The last named are the most 

 primitive of echinoderms. 



The holothurians exhibit a remarkable form of autotomy and regenera- 

 tion. When irritated the whole alimentary canal and the respiratory 

 trees may be thrown out through the mouth, there being developed from 



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Fici. 112. Fig. 113. 



Fig. 112. — A sea cucumber, Cucumaria planci Brandt, from the Mediterranean. 

 From a preserved specimen. X %. 



Fig. 113. — Diagram of the internal anatomy of a sea cucumber, representing the animal 

 laid open and the wall of the body turned to each side. 



the lower branches of the latter a mass of white tough threads in which a 

 possible enemy may become entangled. These structures, however, are 

 soon regenerated. 



238. Crinoidea. — The sea lilies, which were exceedingly abundant in 

 the seas ages ago, are echinoderms which, typically, are attached by the 

 aboral surface to a stalk that rises from the bottom and frequently 

 possesses many rootlike branches (Fig. 114). The oral surface is upper- 

 most and the disc is surrounded by more or less complexly branched rays 

 bearing smaller pinnules arranged like the barbs on a feather. The 



