SERPENT STARS. 



151 



cordifera, a member of a genus nearly related to Aniphiura, is plentiful in the harbor 



of Chai'leston, S. C, and is apparently vivi]iiU'ous, since it may be found with minute 



young clinging to the arms and disc. 



It occurs also in the West Indies. 

 Ophiocymbium cavernosum, taken 



by the ' Challenger ' east of Kerguelen 



Island, is remarkable for the manner 



in which the disc, which is scarcely 



attached to the arms, and is entirely 



covered with small scales, overlies the 



arms "like a Basque cap." Ophio- 



conia cethiops and 0. alexandri, are 



large species from the west coast of 



North America. Among the genera 



with numerous long, usually rough or 



thorny, arm spines the principal are 



Ophiacantha with thirty-nine de- 

 scribed species, and OphiotJirix, with 



fifty-six species. The species of 



Ophiacantha can be readily distin- 



guished from each other by points of ^'^- ''''■ " o^-Haoan^fta cuiy.. 



internal and external structure. The arm spines vary from four to eleven in num- 

 ber at each arm joint. 0. vivipara, which 

 is widely spread in the southern ocean, 

 occurring at depths varying from 20 to 600 

 fathoms, carries its young, till they are quite 

 large, in the ovarial bursa or pouch, whence 

 they often thrust an arm through the gen- 

 ital opening. The bursas are pleated bags 

 having lime scales in their substance and 

 adhering to the thickened wall of the diges- 

 tive cavity. Some of the basal arm spines 

 of OpMomitra dipsacos dredged by the 

 ' Challenger ' off Culebra, West Indies, in 

 390 fathoms, are from five to seven times 

 the length of the arm-joint. The species 

 of OphiotJirix are exceedingly diflicult to 

 distinguish from each other, although the 

 genus is well marked by the very large 

 three-sided radial shields, disc set with 

 thorny grains; five to ten glassy spines 

 (beset with thorns, and often three times as 

 long as the joints), on each arm joint ; single, 

 small, spine-like tentacle scales on each arm- 

 joint ; swollen interbrachial spaces ; and 

 want of perfect union between the halves 



of each mouth-frame. The vertebra of the arms have a peculiar projection which in- 

 terlocks into a slot in the preceding bone, thus giving a fulcrum (says Lyman) for the 



Fig. 133. — Opluothrkc fragUis. 



