260 THE MECHANICAL FUNCTIONS. 



with its feeler, or palpus ; Figures 139, 140, and 141, 

 representing the first, second, and third pair of feet- 

 jaws; and Fig. 142, the first pair of true feet. It 

 would thus seem as if the same constituent element 

 of the fabric is converted by 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 Crus- 

 tacea, the foremost pair of true feet are also modi- 

 fied to suit a particular purpose ; the pincers which 

 terminate them being expanded into a claw, and 

 •constituting a powerful organ of prehension, and 

 a formidable weapon of offence. It resembles a 

 finger and thumb in its power of grasping and 

 strongly compressing the 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 serrated. 



The large portion of shell which is consolidated 

 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 transverse plane. 

 This is the cause of the greater facility the Crab 

 finds in walking side-ways, which it can do with 

 great quickness vi^hen urged by a sense of danger. 



* The labours of Savigny, Auduoin 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 articulated 

 animals, whether belonging to the classes of arachnida, Crustacea, 

 myriapoda, or winged insects. 



