150 



THE MECHANICAL FUNCTIONS. 



number, the advance which this animal can make in any 

 particular direction is excessively slow. 



Besides this movement of creeping, the Asterias is capa- 

 ble of bending and unbending each of its rays: actions, how- 

 ever, which it can perform but very slowly, and not to an 

 extent sufficient to accomplish its removal from one place to 

 another.* 



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 dove-tailed 

 into each other. The 

 form of each piece is that 

 of a lengthened hexagon; 

 and the whole are regular- 

 ly arranged in rows, like 

 a mosaic or tesselated pavement. Ambulacra 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 perfora- 

 tions.t 



On the outer spherical surface of the external crust, there 

 are formed a great number of calcareous tubercles, arranged 

 with beautiful regularity and symmetry in double lines, 

 passing, like meridian circles, fromi the upper to the lower 

 pole of the sphere. Each appears, when magnified, to be a 

 smooth and solid ball, projecting from the surface of one of 



* In adclltioa to these larger tubes, there exists also a smaller set, which 

 pierce the skin in different places, and are channels for the absorption of the 

 water used in respiration. These I shall have occasion to notice more parti- 

 cularly hereafter. 



■\ 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 

 etructure of these organs will be noticed when treating of that function. 



