256 CLASSIFICATION OF ANIMALS 



arms that locomotion is effected. The ambulacral "feet" are 

 without sucking discs and ampullae and do not assist in loco- 

 motion. The arms are almost solid, that is, the digestive and 

 reproductive organs do not extend into them. The skeletal 

 parts are better developed than in the starfish. In the "basket 

 fish" the arms are branched. 



FIG. 146. A brittle star (Ophiuroidea). X 3/4. 



555. Class IV. Echinoidea. In the sea urchins the radii 

 and inter-radii are almost or quite equal and hence there are no 

 "arms." The skeletal parts are broad plates which fit together 

 so as to form a single piece, the shell or test. The oral surface 



