ASTERIAS. 



201 



consistence, are thickly interspersed throughout 

 its texture ; and these, in various parts of the 

 body, both in the upper and the under side, 

 often project from the surface in the form of 

 spines or prickles. They are particularly large 

 around the mouth of the animal, which opens at 

 the centre of the under side. These calcareous 

 masses have a crystalline arrangement, and ex- 

 hibit in fracture the exact oblique angles charac- 

 teristic of the primitive rhomboid of carbonate 

 of lime. 



The under side of each ray (Fig. 95) has a 



96 



groove termed, by Linneus, the ambulacrum, or 

 avenue, a name which it has received from its 

 fancied resemblance to a walk between rows of 

 trees : for each groove contains a quadruple row 

 of perforations, like pin holes, through which 

 small fleshy cylindrical processes pass. These 

 processes extend but a short distance from the 

 surface ; but they admit of being elongated, or 

 retracted, at the pleasure of the animal, by a 

 very curious mechanism, which I shall presently 

 describe. By bending them on either side, in 



