286 THE MECHANICAL FUNCTIONS. 



be impossible, in so brief an outline as the one t 

 am now tracing, to enlarge upon so fertile a 

 topic, without being led too far from the object 

 I have at present more particularly in view, 

 namely the developement of organization with 

 reference to the organs of progressive motion. 



§ 4. Crustacea. 



The plan which Nature appears to have coin- 

 menced in the construction of the Arachnida, is 

 farther pursued in that of the Crustacea. The 

 portions into which the external frame-work of 

 the body was divided in the former,' are still 

 further consolidated in the latter : they are 

 composed of denser materials, and endowed 

 with greater rigidity; thus not only offering more 

 resistance to external forces, but also giving a 

 lirmer purchase to the muscles which are the 

 moving powers. The limbs, as well as the 

 whole body, are encased in tubes of solid car- 

 bonate of lime : they are articulated with great 

 care, and almost always compose hinge joints. 

 The muscles, by which these solid levers are 

 moved, are lodged in the interior, and their 

 fibres either pass directly from one point to 

 another, across the joint; or else they are at- 

 tached to cartilaginous plates, which, for the 

 purpose of receiving the muscles, are made to 



