148 



THE MECHANICAL FUNCTIONS. 



membranous walls, which impart mechanical firmness to the 

 fabric. 



The Jisterias, or star-fish (Fig. SS,) is so named from its 

 star-like form; and the number of rays composing the star 

 is generally five. Besides the tough coriaceous integument, 

 which protects the mass of the body, each ray is farther sup- 

 ported by a series of calcareous pieces, resembling those 

 which compose the spinal column of vertebrated animals,and 

 forming an articulated axis, constructed with the evident de- 

 sign of combining the greatest strength with a proper degree 

 of flexibility. Cartilaginous plates are also added for the more 

 special support of the integument. This integument itself 

 is irritable, and has the power of changing its form, although 

 the muscular fibres by w^hich its motions are effected are not 

 easily distinguished. Calcareous grains, of a solid consist- 

 ence, 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 arrange- 

 ment, and exhibit in 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, termed, 



95 



9S 



by Linneus, the ambulactnim, 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 



