306 



E CHINODERMS. 



effects its purpose by first twisting over one or two of the rays and catching- hold 

 of the ground by the suckers. It then gradually turns over the rest of the bod}'. 

 Cribrella rights itself in the same way as Asterias, but, apparently because of the 

 stiffness of its skeleton, takes much longer over the process. Star-fish, like other 



echinoderms, are a sociable class. 

 Even the deep-sea forms sometimes 

 live in swarms. Many shallow-water 

 forms also are gregarious, and some 

 species have been observed to pair at 

 the breeding- season. The deep-sea 

 star - fish, writes Alcock, " subsist 

 largely on molluscs, the shells of 

 which, along with the chitinous 

 remains of prawns and amphipods, 

 are often to be found in their 

 stomachs ; but some of the character- 

 istic deep-sea forms appear to gorge 

 themselves with globigerina-ooze." 

 The shallow-water forms prefer hard 

 ground, rocks, reefs, or beds of hard 

 sand, where they find in abundance 

 the molluscs and small crustaceans 

 on which they feed. 

 Between the Asteroidea and Ophiuroidea, the family Brisingidce has been 

 considered by some a link ; but in all essential features of structure they agree 

 with the Asteroids. Superficially they resemble the Ophiuroids in having long, 

 thin, flexible arms, clearly distinguished from the small central disc or body. 



STAR-FISH TURNING OVER. 



The Brittle-Stars, — Class Ophiuroidea. 



The name Ophiuroidea, given to the brittle-stars, refers to their long serpent- 

 like arms, which are attached to a relatively small and usually rounded body or disc. 

 The digestive and generative systems do not extend into the arms, but are confined 

 to the body ; so that the arms are appendages to the body, rather than portions of 

 it. They are cylindrical, and have no groove on the under side, such as exists in 

 star-fish, but have little openings through which the tube-feet pass. In this class 

 it is the arms themselves, and not the tube-feet that are used for locomotion. The 

 tube-feet accordingly have no terminal suckers, but are very sensitive to touch, 

 and probably assist respiration. The greater part of each arm is formed by a 

 central axis of successive calcareous segments, not unlike the vertebrae of a back- 

 bone. Each arm-ossicle or vertebra is, however, composed of two parts, one on 

 either side, and united in the middle line ; the successive ossicles being connected 

 by pairs of strong muscular bundles, and articulating with one another by tenon- 

 and-mortice joints. According to the degree of development of these joints, the 

 arms have varied powers of coiling. Thus, in the Claclophiurai, the ossicles have 

 more or less saddle-shaped faces, so that the arms can be twisted round foreign 



