PHYLUM ECHINODERMATA 



205 



5. Crinoidea (kri noi' de a; F., krinon, lily, and eidos, form). — The 

 feather stars and sea lilies. 



236. Asteroidea. — The general characteristics of this class are illus- 

 trated by the starfish. The bases of the rays take up the entire circum- 

 ference of the disc and thus are not definitely marked off from it. The 

 number of rays varies in different species from 5, which is the most usual 

 number, to more than 40. Though usually an odd number, it is not 



Fig. 117. — The long-armed brittle or serpent star, Amphiodia occidental is (Lyman), 

 occurs along the Pacific Coast from California to Alaska. It may voluntarily part with 

 one or more arms — this is autotomy — and regenerate new ones readily. {Drawn by 

 Robert Allen Wolcott.) 



invariably such, since there are forms in which the number is regularly 

 six. In some the disc is small and the rays are long and slender; in 

 others the disc is large and the rays short and broad. This shortening 

 and broadening of the rays may go so far as to produce pentagonal types. 

 Starfishes are rather generally distributed, being absent only from the 

 polar regions. 



237. Ophiuroidea. — This class differs from the preceding in that it 

 possesses slender rays sharply marked off from the disc and in that the 



