204 



THE MECHANICAL FUNCTIONS. 



employed in its construction, instead of forming 

 isolated grains, is accumulated and extended 



into polygonal plates 

 (Fig. 98), the edges 

 of which are dove- 

 tailed 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. Ambu- 

 lacra 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 con- 

 taining, like those of the Asterias, a double row 

 of perforations.* 



On the outer spherical surface of the ex- 

 ternal crust, there are formed a great number 

 of calcareous tubercles, arranged with beautiful 

 regularity and symmetry in double lines, pass- 

 ing, like meridian circles, from the upper to the 

 lower pole of the sphere. Each appears, when 

 magnified, to be a smooth and solid ball, pro- 

 jecting from the surface of one of the polygonal 

 plates of the crust. These balls serve for the 



* An architecture of a still more curious description is exhibited 

 in the calcareous frame-work that 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. 



