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, resembhng 

 those Avhich 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 4l 

 fibres by which its motions are effected are not 

 easily distinguished. Calcareous grains, of a solid 



