There are some species, such as the Crabs and the Brachyura generally, in which the carapace presents a considerable expansion on either side, forming two large compartments in which the greater mass of the thoracic vis cera is contained. Under these circumstances it would be impossible for the animal to escape from its dorsal covering by the relatively in considemble opening which this part presents on its inferior aspect. This renders it neces sary that the carapace, instead of being cast off by simply rising in a single piece, should give way and separate in some direction or another, and this it does by splitting along the curved lines, extending on either side from the mouth to the origin of the abdomen, in the course of which the epimeml pieces cohere with the dorsal one.* The time occupied in the business of throw ing off the shell varies considerably in dif ferent species ; it is also dependent on at mospheric influences. It is the same also, in regard to the number of days necessary to give to the new epidermic layer the consistency of the old tegumentary covering. A general remark, however, and one which is applicable to the whole of the species that have been duly observed, especially those that are found along the shores of France, is this,—that the period which precedes as well as that which follows the change of the shell is one of rest lessness and evident illness. The muscles of these creatures are then flaccid, the flesh is soft and watery,' and as food they are rejected as tasteless and held unwholesome. This would not appear to be the case with the Land-crab, however, according to the statements of several travellers, who inform us that the flesh of this species is never in greater perfection than during the season of the moult.
A phenomenon, which has some analogy with the renovation of the tegumentary skeleton, but which is much more curious, is the repro duction of the legs of these animals. Most Crustacea cast off their claws very easily and without apparent pain ; the separation always takes place in a determinate point near the basis of the member (in the second articula tion), and is soon followed by the formation of a cicatrice, from the surface of which sprouts out a small cylindrical appendage ; this shortly after presents distinct articulations, and re sembles in miniature the organ it is destined to form, but its growth is slow, and it does not for some time attain its full size. If one of the limbs be severed in any other part, the wound continues to bleed, and no renovating process begins unless the animal, by- a violent muscular contraction, succeeds in breaking off the stump in the articulation above mentioned.
The kind of solid sheath formed by the tegumentary skeleton of the Crustacea, and which includes in its interior the whole of the viscera and other soft parts of these ani mals required to be so constructed as not to oppose locomotion ; consequently there exist, either between the different rings of the body or the various constituent elements of the limbs, articulations destined to admit of mo tion to a greater or less extent, between these different pieces. The structure of these arti
culations is of the most simple kind ; the moveable piece rests upon that which precedes it by two hinge-like- joints situated at the two extremities of a line perpendicular to the plane in which the motion takes place. In the in ternal portion of the edge of the moveable piece comprised between the joints, there exists a notch of greater or less depth, destined to admit of flexion, whilst on the opposite or external side, the same edge generally glides under that of the preceding piece. This kind of articulation, whilst it is the most favourable to precision of movement and to strength, has the disadvantage of admitting motion in one plane only ; therefore the whole of the rings of the body, the axis of motion being entirely parallel, cannot move save in a vertical plane; but nature has introduced a kind of corrective of this disadvantage in the structure of the limbs, by changing the directions of the arti cular axes, whence ensues the possibility of general motions being performed in every di rection. Between the two fixed points two opposed empty spaces are observed, left by the rings severally, and destined to admit of the occurrence of motions of flexion and ex tension. The tegumentary membrane which fills it never becomes encrusted or calcareous, but always continues soft and flexible.
The tegumentary skeleton, of which we have thus taken a summary view, supplies the apparatus of locomotion with fixed points of action as well as with the levers necessary to motion. The immediate or active organs of this apparatus are the muscles, the colour of which is white, and the structure of which presents no peculiarity worthy of notice. They are attached to the pieces which they are re quired to move either immediately, or by the intermedium of horny or calcareous tendons, which are implanted upon the edge of the segment to which they belong. To the fixed point they are most commonly at tached immediately. 'Their structure is sim ple, and each segment, in fact, as has al ready been said, being contrived to move in one fixed and determinate plane, the mus cles which communicate motion to it, can constitute no more than two systems anta gonists to each other, the one acting in the sense of flexion, by which the segment moved is approximated to that which precedes it, the other in the sense of extension, by which the segment is brought into the position most remote from the centre of motion. The mus cles that produce these opposite effects, as might have been concluded, are found im planted into the opposite arms of the lever upon which their energy is expended.