CIIUSTACEA. 257 



§ 4. Crustacea. 



The plan which Nature appears to have commenced 

 in the construction of the Arachnida, is farther 

 advanced in that of the Crustacea. The portions 

 into which the external framework of the body 

 was divided in the former are still farther consoli- 

 dated in the latter : they are composed of denser 

 materials, and endowed wath greater rigidity ; thus 

 not only offering more resistance to external forces, 

 but also giving a firmer purchase to the muscles 

 which are the moving powers. The limbs, as well 

 as the whole body, are encased in tubes of solid 

 carbonate of lime; they are articulated with great 

 accuracy, 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 attached to cartilaginous 

 plates, which, for the purpose of receiving the 

 muscles, are made to project into the interior of the 

 upper portion of the limb, being themselves im- 

 movably connected with the lower portion. By 

 this expedient, not only is the employment of a 

 tendon dispensed with, but a larger surface is pre- 

 sented for the attachment of the muscles, which, 

 by acting also upon a longer lever, obtain great 

 mechanical advantage. It would be superfluous to 

 occupy more time in explaining the minutiae of 

 structure in these joints, because the simple inspec- 

 tion of the limbs of a crab or lobster will give clearer 

 ideas of this mechanism than can be conveyed by 

 VOL. 1. s 



