102 



THE MECHANICAL FUNCTIONS. 



careous covering, there is also provided, lor the 

 support of the softer parts, a kind of internal ske- 

 leton, or jointed framework. The organs in the 



interior of the body are farther supported by mem- 

 branous 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 supported by 

 a series of calcareous pieces, resembling those 

 which compose the spinal column of vertebrated 

 animals, and forming an articulated axis, con- 

 structed with the evident design of combining great 

 strength with a proper degree of flexibility. Car- 

 tilaginous 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 



