ECHINOIDEA 59 



to that of the starfish. Locomotion is very slow and is per- 

 formed by the tube-feet, aided by the long spines. 



The pedicellariae are similar to those of the starfish, but are 

 more fully developed, having three pinchers instead of two. 



The food consists largely of green algce and brown seaweed, 

 for the sea-urchin is a vegetable feeder, though it eats small 

 marine animals also. 



Digestive System. There are five hard white teeth with 

 which they gnaw their food. These teeth are connected with a 



Fig. 45. Strongylocentrotus drobachiensis. Oral view, showing spines, 

 " feet," and teeth. (Clark, in Bulletin 550, U. S. F. C., 1902.) 



complicated calcareous framework under muscular control. 

 The whole apparatus is called " Aristotle's lantern." 



The intestines are long, coiling about two and a half to three 

 times, instead of being short like those of the carnivorous star- 

 fish. 



The hepatic caeca and gastric pouches are absent. This 

 lack, as well as the structure of the mouth parts and the long 

 coiled intestine, correlates with the feeding habits of these 

 herbivorous animals. 



