3°6 
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 body. 
Cribrella rights itself in the same way as A.sterias, 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 Opliiuroidea, 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. 
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 vertebras 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 Cladophiurce, the ossicles have 
more or less saddle-shaped faces, so that the. arms can be twisted round foreign 
A 
STAR-FISH TURNING OVER. 
