form that for which the utmost effort of its muscular powers appears to be requisite. Being unwilling to propagate a method of adding to the torments of animals for the mere gratification of that cruel curiosity in which physiologists have too often, and with justice, been accused of indulging, I will not here communicate the method by which the animals of this tribe may be induced to perform this voluntary mutilation; but it is rather too well known to children who have been brought up on sea-shores.

It is, nevertheless, doubtful whether the effort itself is attended with pain, whatever the previous torment may be, as it seems often to be done without any apparent motive. The whole tribe is of an extremely ferocious and irritable character; and with many, particularly of the younger animals, the mere attempt to take them, even without actual contact, causes them to drop the two hands, or larger claws. Others do it if confined in a box or a glass of water; and almost all of the smaller kinds, or half grown ones, part with any claw by which it is attempted to retain them.

To render the description of this process intelligible, it will be necessary to understand the general structure of the limbs of this tribe. The accompanying sketch, (Fig. 1, Plate I.) is from one of the small claws of the spider crab, being the species just at this moment under my eye. The structure of the several parts, as well as the form of the limb, is of course somewhat peculiar to this species; but the difference is not material among the whole, and the same explanation and drawing will, with slight modifications, apply to all the instances in which it occurs. Even if I had not accidentally had this species at hand, I should have preferred it; as the structure of the suture where the separation takes place, is rather more distinct and remarkable than in the other species which I have examined.

The limb of a crab consists of six parts, each of which has an appropriate motion in two directions, by means of a pair of muscles adapted to the nature and extent of the space to be described. In the extreme joint, the motion of direct flexion, towards the body, is considerable; but the extension is limited by the structure of

the shell; and these motions are produced by two long muscles occupying the second phalanx. The motion of this phalanx upon the third, is lateral in two directions, and tolerably extensive, but less so than any of the direct motions. It is performed by a pair of muscles of considerable power, which lie in the third phalanx, The motion of this on the fourth, is a motion of extension and flexion, like that of the first; the latter being, in the same manner, considerable, while the former is limited; and the muscles that produce them are of considerable power, as occupying the great length of the fourth phalanx.

The fifth phalanx is very short, and is articulated to the fourth in an oblique manner, by a very narrow ligament; while the lower shell surmounts the upper one in such a manner as to limit the motion of the latter on the former to a very minute space laterally, which is also somewhat more considerable in one direction than another, but which varies, in this respect, in different species of crabs. These lateral motions are produced by two broad, but very short, muscles; as it will be shortly seen that but a small part of this fifth phalanx can be allowed for them, a great part of it being allotted to the arrangement provided for detaching the limb,

The sixth, or last, phalanx, is extremely short, and is articulated by a somewhat lax ligament to the preceding, which admits also of a direct motion like those of the first and third; but which is, on the contrary, much more free and extensive backwards than forwards. This phalanx is also connected with the former, by short and feeble muscles; but the principal motions in this fifth phalanx, which are amongst the most powerful in the limb, are produced by two long muscles, a flexor and an extensor, which pass quite through the sixth phalanx, and are inserted within the body of the animal. The fifth phalanx therefore moves immediately on the body, independently of the sixth, or is, at least, capable of so doing,

The last, or sixth phalanx, is connected with the body by a very wide ligament, admitting of a very considerable lateral motion, but more extensive inwards than outwards, and allowing the limbs

to be folded under the body. Some short muscles are appointed to this office; but it is unnecessary to enter into further particulars; as enough is given in this sketch of the motions of the limb and the position of the muscles, to illustrate the object of this paper. The beauty of the mechanism, by which strength of articulation is combined with extent and facility of motion, cannot fail to strike the most negligent observer of nature.

It is in the fifth phalanx that the provision for detaching the limb is placed, and the accompanying sketches will assist in rendering the description intelligible.

It will be perceived that, on the exterior side of this phalanx, there is a pale ring, transversely drawn round it (Plate I. Fig. 1.); and, with a magnifying glass, it is easy to perceive, though much more distinctly in some species than in others, that a very fine line lies in the middle of it, not exactly in one plane, but slightly undulated. (Plate II. Fig. 6.) If the edge of a knife be forced upon this line, it will be found that the phalanx, at the moment it yields, flies asunder with a loud crack. In every part of the shell, the bony matter is deposited in a fibrous manner, transversely to the plate; but at this particular part, the fibres are peculiarly fine and straight, while the structure is also more brittle or tender and the colour paler, (Plate II. Fig. s.) The division is, in fact, a natural suture; nor is it possible to separate the joint in any other place than where this exists.

If now a longitudinal section of the whole phalanx be formed, the appearance represented in Fig. 7 will be seen. It will be observed, that the suture occupies a thinner part of the shell, or that this portion is more slender than the general shell of the limb, and that it is bounded on each side by two reinforced rings. This is the case at least with the spider crab; but it is less visible in some other species which I have examined, although the peculiar structure of the suture, in other respects, is always to be distinguished. I must nevertheless remark, that in some, as in the lobster and crawfish, the external ring is by no means conspicuous, though the same provision exists in the internal arrangement, nor is it very obvious, externally, in the cancer pagurus. In the

same figure, there is a rude representation of the position and insertion 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 ses, 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 explanation, by considering the structure above described, and the

positions and actions of the last pair of flexor and extensor muscles; and thus it has been attempted to explain it. It is obvious, however, on a moment's consideration, that no actions of these muscles, however powerful, could alone produce the consequence in question; as they could have no further effect than that of fixing that part of the fifth phalanx, which is below the suture, more firmly on the body. To explain the mode in which their action is rendered efficient towards the separation of the limb, it is necessary to describe the forms and relations of the fifth and sixth phalanges more particularly; as, in these, the true solution of the difficulty will be found. It is from inattention to these important circumstances, that this process has appeared so mysterious; and assuredly it is difficult, on a first view of the operation as performed by the animal, to witness it, without surprise at the facility with which it is effected, and at the apparent inadequacy of any conceivable means for producing the separation. The obvious effect of muscular action, is to approximate the insertions of the muscles; yet here it appears to act as if it was to separate them.

As it is impossible to render this structure intelligible without drawings, some sketches are added for that purpose; and, as being more obvious in the common crab, (Cancer pagurus,) they are taken from that species. They are limited to the fourth, fifth, and sixth phalanges; being the only ones required for illustrating the subject; and these are numbered, so as to correspond to Fig. 1, Plate I. The place of the suture is indicated. in these sketches, as it is not so defined externally as in the spider crab. In Plate II. Fig. 5, is an outside view of part of the limb in a state of flexion; the state of extension being performed by the approximation of the points A, B, until they meet, so as that the indicating lines coincide in a common line C. In Fig. 2, Plate I. the phalanx is in a half-extended state, and is further so turned, as to shew more distinctly the forms of the two phalanges at A and B; while, in Fig. 3, Plate I. the extension being completed, the points A, B, are brought into absolute contact at C. Another view of the meeting of those points when in a state of extension,