200 THE MECHANICAL FUNCTIONS. 



retain, in a very marked manner, the radiated 

 disposition so characteristic of Zoophytes : for 

 we find all their parts symmetrically arranged 

 either in lines, or in compartments, which pro- 

 ceed from a common centre, or axis, and which 

 are repeated, in regular succession, all round the 

 circumference (See Fig. 88 to 94). Besides an 

 external horny, or semi-calcareous covering, there 

 is also provided, for the support of the softer 

 parts, a kind of internal skeleton, or jointed 

 frame-work. The organs in the interior of the 

 body are further supported by membranous walls, 

 which impart mechanical firmness to the fabric. 

 The Asterias, or star-fish (Fig. 88), 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 verte- 

 brated animals, and forming an articulated axis, 

 constructed with the evident design of combining 

 the greatest strength with a proper degree of flexi- 

 bility. 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 which its motions are effected are not 

 easily distinguished. Calcareous grains, of a solid 



