ECHINUS. 



185 



this animal can make in any particular direction is 

 excessively slow. 



Besides this movement of creeping, the Asterias 

 is capable of bending or unbending each of its rays, 

 and of bringing them nearer to each other ; actions, 

 however, which it can perform but very slowly, 

 and not to an extent sufficient to accomplish its 

 removal from one place to another, but which may 

 enable it to adapt its form to the passages through 

 which it may be creeping. 



The skeleton of the Echinus, or sea-urchin, (Fig. 

 91), is still more artificially framed than that of the 

 Asterias. It has a spheroidal form, like that of an 

 orange ; the calcareous material employed in its 

 construction, instead of forming isolated grains, is 

 accumulated and extended into polygonal plates 



(Fig. 98), the edges of 

 which are dovetailed 

 into each other. The 

 form of each piece is 

 that of a lengthened 

 hexagon ; and the whole 

 are regularly arranged 

 in rows, like a mosaic or tesselated pavement. Am- 

 bulacra are also seen on the surface of the shell, 

 passing vertically down the sides of the sphere, 

 similar to the meridians of a globe ; and containing, 

 like those of the Asterias, a double row of per- 

 forations.* 



* An architecture of a still more curious description is exhibited in 

 the calcareous framework which has been provided for the support 

 of the teeth, and other organs of mastication, with which this animal 

 is furnished. The structure of these organs will be noticed when 

 treating of that function. 



