Animal and vegetable physiology, considered with reference to natural theology, by Peter Mark Roget .. . ither pass directly from one point toanother, across the joint; or else they are at-tached to cartilaginous plates, which, for thepurpose of receiving the muscles, are made to CRUSTACEA. 287 project into the interior of the upper portion ofthe limb, being themselves immovably connectedwith the lower portion. By this expedient, notonly is the employment of a tendon dispensedwith, but a larger surface is presented for theattachment of the muscles, which by actingalso upon a longer lever, obta
Animal and vegetable physiology, considered with reference to natural theology, by Peter Mark Roget .. . ither pass directly from one point toanother, across the joint; or else they are at-tached to cartilaginous plates, which, for thepurpose of receiving the muscles, are made to CRUSTACEA. 287 project into the interior of the upper portion ofthe limb, being themselves immovably connectedwith the lower portion. By this expedient, notonly is the employment of a tendon dispensedwith, but a larger surface is presented for theattachment of the muscles, which by actingalso upon a longer lever, obtain great mecha-nical advantage. It would be superfluous tooccupy more time in explaining the minutiae of; structure in these joints, because the simple in-spection of the limbs of a crab or lobster willgive clearer ideas of this mechanism than canbe conveyed by any laboured description. Wemust content ourselves with a brief sketch ofthe principal constituent parts of these externalmembers of the Crustacea. The number of pairs of legs is either three orfour : each leg is divided into five pieces. The. piece H (Fig. 137), next the trunk, is termedthe haunch, to which is united the trochanter,T; after which come, in succession, the femur 288 THE MECHANICAL FUNCTIONS, or thighy F ; two portions of the leg, l ; andthe tarsus, p. The haunch is usually short,being interposed merely as a base for increasingthe extent of motion of the pieces which follow :and sometimes it is itself composed of more thanone piece. The leg is usually divided into twopieces, by a joint. The tarsus is terminated bya single or double hook, and sometimes by apincer, or claw. New organs, not met with among the Arach-nida, are here for the first time developed,namely, the AntenncB, of which there is one oneach side of the head. They are denominated,in popular language, the feelers; although it ismore than probable that they perform somefunction of higher importance than that of con-veying perceptions of mere to
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