SERPENT STARS. 



147 



spirnlly in such a way as to oiulosc a closed chamber between them. The ]iinnulcs are 

 formed of broad, flat joints, and can be spirally rolled towards the arms. 



In Ilolupiis there is a marked divi- 

 ■sion into Viivium and tri\ ium, as in some 

 holotliurians, the three facets of the 

 triviiim arc lai'gcr than the other two 

 facets (the central one largest), and the 

 three arms attached to those facets of 

 the cup are larger than those joined to 

 the opposite side. Another attached 

 genus is jBat/njrri)ii(g, one species of 

 which was dredged with fli/ocriiius in 

 eighteen hundred an<l fifty fathoms, off 

 Brazil, while another was taken at two 

 thousand four Imudred and thirty-five 

 fathoms in tiie Uay of Biscay. 



CLA.S.S II. — STELJ.EKIDA. 



The Stellerida are echinoderms with 

 star-shaped or pentagonal bodies, a well- 

 developed water system, and an internal 

 skeleton, consisting of ambulacral ]ilates 

 which arc different from those of the 

 Kchinoidea, since the nerve cor<ls and 

 radial ambulacra! vessels lie outside and 

 below them. There art' two orders, 

 Asteroidea and Ophiuvoide;!. 



OitDKit I. — Ol'lIIUKOIDEA. 



The Ophiuroidea are ;i group of star- 

 fishes, characterize<l by a nioi'e or less 

 sharply-defined central disk, containino- 

 a digestive cavity, whi('h may be simple 

 or much plaited, but which 'ihies not 

 pass into the arms, '{'here is no anal 

 ojiening. The arms have an a.\is com- 

 posed (if calcareous ossicles, usually 

 called arm-bones, which greatly resem- 

 ble vertebrae, and each of Avliich is 

 made up of two sections. These two 

 sections rei)resent the ambulacral plates of ordinary star-fishes. The a.xis is eased either 

 with plates, or with a thick skin having rudimentary plates beneath, and the plates upon 

 the sides of the arms usually bear spines. Within the hollow of the arm, covered by 

 the under arm-plates, yet in the same relative position to each other and to the ambu- 

 lacral plates or arm-bones, that obtains in the true star-fishes, run the nerve, the neuval 

 canal, and the ambulacral vessel of the water system. There are no araiJullag in con- 



- Bathl/r 



