258 



THE MECHANICAL FUNCTIONS. 



any laboured description. I shall, therefore, only 

 give 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. K37), next the trunk, is termed the haunch, 

 to which is united the trochanter (t) ; after which 

 come, in succession, the femur or thigh (f) ; two 

 portions of the leg (l) ; and the tarsus (p). The 

 haunch is usually short, being interposed merely 

 as a base for increasing the extent of motion of the 

 pieces which follow ; and sometimes it is itself com- 

 posed of more than one piece. The leg is usually 

 divided by a joint into two pieces. The tarsus is 

 terminated by a single or double hook, and some- 

 times by a pincer, or claw. 



New organs, not met with among the Arachnida, 

 are here for the first time developed, namely, the 

 Antennce, of which there is one on each side of the 

 head. They are denominated, in popular language, 

 the feelers; although it is more than probable that 

 they perform some function of higher importance 

 than that of conveying perceptions of mere touch. 

 The antennae consist of slender filaments, composed 



