142 “ANIMAL FORMS 
skeleton indicate that the sea-urchin is comparable to a 
starfish, with its dorsal surface reduced to insignificant 
proportions. 
In the sea-urchins the calcareous plates possess a great 
regularity, and are so closely interlocked that they prevent 
e £ 
Fie. 88.—Brittle- or serpent-stars (species undetermined), Natural size. 
any motion of the body-wall. Also, each plate is usually 
provided with highly developed spines, movable upon a ball- 
and-socket joint. These spines serve for locomotion, and, 
in some instances, for conveying food to the mouth. A 
considerable number of sea-urchins show an irregularity in 
form which destroys to a corresponding degree the radial 
symmetry. This is due to various causes, but especially to 
a compression of the body, which, in the “sand-dollars,” 
