by which Crabs throw off their Claws. 9 



in either direction. The muscles, by which it is effected, are broad, but 

 they arise from above the suture, or white ring 1 , in the fifth phalanx, and 

 are consequently very short. 



The fifth articulation, lying between the fifth and sixth phalanges is 

 very free, and admits of direct motion both ways. But it differs from all 

 the preceding in the great extent of the motion backwards ; by which the 

 whole limb admits of being thrown back, so as to be parallel to the flat 

 surface of the animal. It has been already seen that this is the motion 

 which prepares the limb from being detached, and the means by which 

 this, is effected will be more fully- shown in the following figures. The 

 mnscles by which the flexion and extension are performed, and which also 

 by their action serve to separate the limb, are not, as in the former cases, 

 inserted in the next, or sixth, phalanx, but pass quite through it to be 

 fixed in the shell of the thorax. Their origin is below the white ring, or 

 suture, as that of the last pair was above it. 



The sixth and last phalanx is articulated to the body by a very wide 

 and lax ligament, admitting, however, only of a lateral motion, which is 

 performed by muscles inserted in its sides, and fixed at the other extremity 

 to the shell of the body. 



Plate I, Fig. 2. Represents the sixth and fifth phalanges, with part of 

 the fourth, in a limb of the Cancer Pagurus. The place of the suture, in 

 the fifth phalanx, is less marked in this species ; and it was for that reason 

 that the spider crab was preferred for the purpose of showing that part. 

 It is, however, indicated in the. figure. The limb is here thrown back by 

 the action of the extensor; and both the muscles are introduced, as sepa- 

 rated from the body and passing through the sixth phalanx. The pro- 

 tuberance of the fifth phalanx, which, in the act of detaching the limb, is 

 brought to re : st on the sixth at B, is shown at A. 



Plate I, Fig. 3. In this figure another view of the:same parts is given, 

 for the purpose of shewing the complete extension of the limb at the 

 moment when it is to be detached. At the point C, the protuberance of 

 the fifth phalanx at A, bears strongly on the. point B in the sixth, or A 

 and B coincide at C. 



Plate I, Fig. 4. Is a direct view of the interior of the limb at the same 

 place, when in a state of extension. This figure is given principally to 

 show the suture, which is very visible in the interior of the fifth phalanx ; 

 while it also represents the bearing of the fifth and sixth on each other, on 

 the opposite side at C. 



Plate II, Fig. 5. The same parts are here represented in a state of 

 moderate flexion, for the ; purpose of shewing more distinctly the protu- 



