by which Crabs throw off their Claws. 7 



is seen in Fig. 4, where a front view of the limb is given ; and 

 here also the coincidence of the two protuberances A, B, as the 

 point of mutual contact, C, is indicated. 



In all the figures, the position of the suture is marked; and, in 

 Figs. 2 and 3, an attempt is also made to shew the positions of the 

 flexor and extensor muscles of this phalanx. These, passing 

 through the last, or the sixth, phalanx, are inserted in the body of 

 the shell below, and beneath the suture of the fifth phalanx 

 above, and it is by them that the action of separation is effected. 

 But the mode of action will be more easily understood, by simpli- 

 fying the appearance of the parts ; as is attempted in that which 

 is rather a diagram than a drawing, at Fig. 9, Plate II. In this, 

 the shell is supposed transparent, to shew the directions and in- 

 sertions of the separator muscles. 



The limb being firmly extended, the protuberance A is thus 

 brought to rest firmly on B at the point C, and in such a 

 manner, that the lower portion of the suture itself just touches 

 the sixth phalanx. If the whole of the fifth phalanx, above the 

 suture, could be thus supported on the sixth, it is evident that the 

 contraction of the flexor and extensor muscles would draw the 

 lower portion of the former towards the shell or body, the latter 

 phalanx being at the same time pressed against it, and thus tend 

 to separate the lower from the upper part of the fifth phalanx. 

 But the necessary freedom of the motion of flexion would not 

 admit of such a construction ; and the effect is therefore pro- 

 duced in a different manner. 



For this purpose, the chief operation of the extensor muscle is, 

 as before remarked, to fix the protuberance A firmly on B, which 

 thus becomes a fulcrum, or point, on which the upper portion 

 tends to revolve when the flexor is brought into action. Thus 

 the flexor muscle acquires a lever of considerable power; and, 

 on being caused to contract, it draws the lower portion of the 

 fifth phalanx from the upper, and effects the separation. That 

 this is the efficient cause, is rendered evident by the crack com- 

 mencing at D ; although the whole is completed so instantane- 

 ously, that it requires a quick eye to perceive a difference between 



