380 RADIATA. 



ball-and-socket joint, so as to turn in various di- 

 rections. Between these two rows run down two 

 channels, perforated with hundreds of minute holes, 

 through which are protruded or drawn back the 

 sucker-feet. Multitudes of smaller spines project 

 in all directions from the surface of the skin, be- 

 sides the larger ones above mentioned. Every seg- 

 ment is composed of many pieces of a regular six- 

 sided figure, which are dovetailed into each other 

 with beautiful precision ; an admirable arrangement, 

 by which alone the growth of the animal could be 

 permitted. For if the shell were composed of one 

 continuous piece, formed as it is by deposition from 

 the surface of the animal, it is plain that every 

 layer deposited would diminish the interior, while 

 the contained animal would be ever increasing in 

 size. But, as it is, every one of these six-sided 

 pieces is increased by layers on its inner surface, 

 each layer being a little wider than the preceding ; 

 and thus each piece gradually enlarges, (and conse- 

 quently the whole shell,) while at the same time 

 the definite form of each is exactly retained. On 

 the under surface of this flattened sphere, in the 

 point where all the divisions meet, as in one of the 

 poles, is situated the mouth, curiously furnished 

 with five hard and sharp teeth, somewhat like the 

 incisors of a Squirrel, the points of which meet 

 in the centre. The food of these creatures is al- 

 together animal; and, though the Sea-urchin does 

 not seem at first sight very well qualified to pursue or 

 to seize prey, yet its organs doubtless correspond to 



