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 



FIG. 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," 



