SERPENT STARS. 



147 



Spirally in such a way as to enclose a closed chamber between them. The pinnules are 

 foi-med of broad, flat joints, and can be spirally rolled towards the arms. 



In Holopus there is a marked divi- 

 sion into bivium and triviura, as in some 

 holothurians, the three facets of the 

 trivium are larger 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 JiathycHmis, one species of 

 which was dredged with Ilyocrinus in 

 eighteen hundred and fifty fathoms, off 

 Brazil, while another was taken at two 

 thousand four hundred and thirty-five 

 fathoms in the Bay of Biscay. 



Class II.— STELLBKIDA. 



The Stellerida are echinoderms with 

 star-shaped or pentagonal bodies, a well- 

 developed water system, and an internal 

 skeleton, consisting of ambulacral plates 

 which are different from those of the 

 Echinoidea, since the nerve cords and 

 radial ambulacral vessels lie outside and 

 below them. There are two orders, 

 Asteroidea and Ophiuroidea. 



Okder I. — ophiuroidea. 



The Ophiuroidea are a group of star- 

 fishes, characterized by a more or less 

 sharply-defined central disk, containing 

 a digestive cavity, which may be simple 

 or much plaited, but which does not 

 pass into the arras. There is no anal 

 opening. The arms have an axis com- 

 posed of calcareous ossicles, usually 

 called arm-bones, which greatly resem- 

 ble vertebrae, and each of which is 

 made up of two sections. These two 

 sections represent the ambulacral plates of ordinary star-fishes. The axis is cased 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, Thei'e are no ampullae in con- 



FlG. 129. — Bathycrinus aldrichianus. 



