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. x16,—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 cesophagus 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 Echinide is still imperfectly known; nevertheless, 
