ECHESrODERMATA. 281 



Oil mother ! I wish for one thing only : to be to be without these 

 exterior and compromising appendages ; to be thick-set, strong, and 

 round, for that is the shape in which I should be the least exposed ; 

 in short, to be a centralized being. I have very little instinct for 

 travel. To roll sometimes from the surface to the bottom of the sea is 

 enough of travel for me. Glued firmly to my rock, I could there 

 solve the problem, the solution of which your future favourite, man, 

 seeks for in vain that of safety. To strictly exclude enemies and 

 admit all friends, especially water, air, and light, would, I know, cost 

 me some labour and constant effort. Covered with movable spines, 



Fig. 112. Echinus mamillatus (Lamarck), natural size. 



enemies will avoid me. Now, bristling like a bear, they call me an 

 urchin." 



Let us now look a little more closely at the general structure of the 

 sea-urchins in zoological language, Echinidse. 



The body of the sea-urchin is globular in form, slightly egg-shaped, 

 or of a disk slightly swollen. It consists essentially of an exterior shell 

 or solid carapace, clothed in a slight membrane furnished with vibratile 

 cilia. This carapace is formed of an assemblage of contiguous polygonal 

 plates, adhering together by their edges. Their arrangement is such 

 that the test or shell may be divided into vertical zones, each springing 



