ASTERIAS. 



183 



are effected are not easily distinguished. Calca- 

 reous grains, of a solid 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 particu- 

 larly large around the mouth of the animal, which 

 opens at the centre of the under side. These cal- 

 careous masses have a crystalline arrangement, and 

 exhibit, on fracture, the exact oblique angles cha- 

 racteristic of the primitive rhomboid of carbonate 

 of lime. 



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



95 



96 



termed, by Linneus, the ambulacrum, or avenue, a 

 name which it has received from its fancied resem- 

 blance to a walk between rows of trees : for each 

 groove contains a quadruple row of perforations, 

 like pin holes, through which small fleshy cylin- 

 drical 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 their expanded state, the Asterias 

 is capable of effecting a slow progressive motion ; 

 so that these processes may be regarded as corre- 



