282 



THE OCEAN WORLD. 



proportion to its size, and the teeth are of proportionate dimensions. 

 As these project from a very formidable mouth, one can easily be 

 assured of the sharpness of their extremities by intruding his fingers 

 into them. In fact, it is necessary that these organs should be 

 singularly powerful, because, as we shall see farther on, the sea-urchin 

 possibly makes incisions in the solid rock with them, and hollows out 

 shelter for himself. The strong and sharp teeth grow at the base in 



Fig. 116. Clypeaster rosaceus (Lamarck). 



proportion as they are used at the points, as is the case with some of 

 the rodent mammals. By this means they are always sharp and in 

 good condition. Five groups of powerful muscles are used to work 

 these terrible grinders. 



To this formidable mouth is attached an oesophagus or gullet, and 

 an intestine which extends along the interior walls of the corona, 

 describing the circumference of its principal contour. 



The food of the Echinidae is still imperfectly known ; nevertheless^ 



