290 THE MECHANICAL FUNCTIONS. 



nature into the one or other of these organs, 

 according as best suits the exigencies of each 

 particular case.* 



In the lobster, the crab, and many other 

 analogous Crustacea, the foremost pair of true 

 feet are also modified to suit a particular pur- 

 pose ; the pincers which terminate them being 

 expanded into a claw, and constituting a pow- 

 erful organ of prehension, and a formidable 

 weapon of offence. It resembles a finger and 

 thumb in its power of grasping and strongly 

 compressing any object on which it seizes ; and 

 to enable it to do this with more effect, the inner 

 edges of both parts of the claw are notched or 

 serrated. 



The large portion of shell which is consoli- 

 dated into one piece, and covers the upper part 

 of the body, is termed the shield, or carapace. 

 The tail of the crab is very short, and is united 

 with the body, appearing as if it had been folded 

 under it. The feet-jaws are particularly large, 

 but short : the articulations of the feet are such 

 as to allow of scarcely any motion but in a trans- 

 verse plane. This is the cause of the greater 



* The labours of Savigny, Audouin and Latreille appear to 

 have established a complete analogy in the respective component 

 parts, not only of the feet, feet-jaws, jaws and mandibles, but 

 also of the palpi and other appendices attached to the head, in 

 all the articulated animals, whether belonging to the classes of 

 arachnida, Crustacea, myriapoda, or winged insects. 



