SEA-URCHINS. 



5i 



with retractile suckers, similar to those described in the Star- 

 fishes; and, by the joint action of their spines and suckers, 



Fig. 33. SKA-UUCIIIX (EXTKUIOR;.* 



they can move in any direction they please, or can moor 

 themselves to the surface of sub-marine rocks. 



The calcareous covering of the Sea-urchin exhibits a sin- 

 gular and beautiful contrivance for the progressive growth of 

 the animal. It is not one piece, as the word "shell," so 

 commonly applied to it, would lead us to suppose. It is 

 formed of a multitude of pentagonal pieces, accurately fitted 

 together, some rows of them bearing the tubercles to whHi 

 the spines arc attached, and others pierced with hundreds of 

 minute orifices, through which the tubular suckers are pro- 

 truded. A living membrane, analogous to that found in some 

 of the Polypes, covers the entire surface, and dips down 

 between the several plates. It has the power of depositing a 

 calcareous secretion, which, being added to the edges of the 

 plates, augments all in an equal ratio; and thus, whatever 

 may be the size of the Sea-urchin, the relative proportion of 

 the several parts is uniformly maintained. 



It is impossible to contemplate the admirable mechanism of 

 the spines and suckers, and the elaborate structure of the 

 shell, without at once feeling the conviction that in them we 

 behold a portion of "the works of the Lord, and His wonders 



* Fig. 33. The spines have been removed from the left side for the purpose of 

 exhibiting tho arrangement of tho pieces composing the "shell " underneath. 



