by ivhich Crabs throw off their Claws. 5 



same figure, there is a rude representation of the position and in- 

 sertion of the muscles, as this is necessary for understanding the 

 nature of the action by which the limb is detached. 



It will be seen that the two short muscles which produce the 

 confined lateral motion between the fourth and fifth phalanges, or 

 at A, (in Fig. 1, Plate I.) are inserted above the suture, (Plate II. 

 Fig. 7,) and that the flexor, which alone is visible in this section, 

 and which bends the joint at B, (Plate I. Fig. 1,) is attached below 

 it ; the opposing extensor, not visible in this drawing, because 

 occupying that part of the shell which has been removed, has a 

 similar insertion. Thus there is left a vacant space between the 

 two sets of insertions, on each side of the suture ; and this, 

 according to the species or size of the animal, varies from a 

 quarter to the eighth of an inch. In the living state of the animal, 

 this is filled with a mucilaginous matter, which coagulates on 

 boiling, so as somewhat to resemble the curd of milk; and which, 

 after the voluntary separation of the limb, forms a protection to 

 the ends of the last pair of flexors and extensors, and to the 

 cavity of the joint. In Fig. 6, these muscles, by which the action 

 of detaching the limb is produced, are represented as separated 

 from their inferior insertions within the body ; bearing here the 

 same proportion to the fifth phalanx, as they do in the species 

 from which this drawing was made. 



It is now necessary to remark, that, in proceeding to detach the 

 limb, the animal frequently throws the whole of the limbs into a 

 state of violent extension, remaining perfectly rigid, as if under 

 the operation of a tetanus or universal spasm. In other cases, 

 the injured limb alone is so extended; and it is probable that 

 these differences depend on the state of vigour or debility in the 

 animal. When feeble, this action is often continued for some 

 time, or relaxed, and again renewed, without producing the 

 desired effect. But, when the animal is sufficiently powerful, the 

 limb suddenly drops off at the suture, with a loud crack, in a 

 second or two after the extension. 



This singular process seems, at first sight, to be capable of ex- 

 planation, by considering the structure above described, and the 



