CRUSTACEA. 



287 



project into the interior of the upper portion of 

 the limb, being themselves immovably connected 

 with the lower portion. By this expedient, not 

 only is the employment of a tendon dispensed 

 with, but a larger surface is presented for the 

 attachment of the muscles, which by acting 

 also upon a longer lever, obtain great mecha- 

 nical advantage. It would be superfluous to 

 occupy more time in explaining the minutiae of 

 ; structure in these joints, because the simple in- 

 spection of the limbs of a crab or lobster will 

 give clearer ideas of this mechanism than can 

 be conveyed by any laboured description. We 

 must content ourselves with a brief sketch of 

 the principal constituent parts of these external 

 members of the Crustacea. 



The number of pairs of legs is either three or 

 four : each leg is divided into five pieces. The 



piece H (Fig. 137), next the trunk, is termed 

 the haunch, to which is united the trochanter, 

 T ; after which come, in succession, the femur 



