CRUSTACEA. 



205 



great care, and almost always compose hinge joints. The 

 muscles, by which these solid levers arc moved, are lodged 

 in the interior, and their fd^res 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 intothe interior of the up- 

 per portion of the limb, being themselves immoveably con- 

 nected 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 mechanical 

 advantage. It would be superfluous to occupy more time in 

 explaining the minutiae of structure in these joints, because 

 the simple inspection 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 ii, (Fig. 137,) 



next the trunk, is termed the haunch, to which is united 

 the trochanter, t; after which comes in succession the fe- 

 mur 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 composed of more 



