The motions in flexion tend universally to bring the extremities and the different rings towards the ventral aspect of the body ; it is consequently upon this aspect that the flexor muscles are inserted, and these are in general the more powerful. On the contrary, and in accordance with the nature of the motion pro duced, it is upon the superior or dorsal aspect of the segments that the extensor muscles are attached. In the trunk the two orders of mus cles generally form two distinct layers, the one superficial, the other deep; the former thin and sometimes absent,' the second, on the contrary, very powerful wherever powerful motions are required. The muscles generally extend from the arc above to the one immediately below, passing for the most part from the anterior edge of the upper to the anterior edge of the lower segment. The extent and the direction of the flexion of which any segment is su,s ceptible, depend on the size of the inter annular spaces above or below the ginglymoid points; and as these spaces are in general of considerable magnitude on the ventral aspect, whilst the superior arcs are in contact and can only ride one over another in a greater or less degree, it is only downwards that the body can be bent upon itself; while upwards, or in the sense of extension, it can hardly in general be brought into the horizontal line.
Thus far what has been said applies more especially to the rings of the body, but the extremities present nothing that is essentially different either as regards the mode in which the tubular segments are articulated to one another, or as regards the mode in which the muscles are inserted. Each of these indeed having but one kind of motion, and even that very limited in its extent, nature has aided the deficiency, as has been stated, by increasing the number of articulations, by which extent of motion is conferred, and in varying the direction of the articular axes, an arrangement by which the animal obtains the ability of moving in every direction, but at the expense both of power, ra pidity, and precision in its motions. Each. seg ment of a limb encloses the muscles destined to move that segment which succeeds it, un less it be too short and weak for this end, in which case the muscles themselves have their origin at some point nearer to the median plane of the body. As a general law the muscles are observed to be more powerful in proportion as they are nearer to the centre, which is to be explained by the fact that each motion they then communicate is transmitted to a larger portion of a limb, to a lever longer in that sense in which it is disadvantageous • to the power. Occasionally, however, the two last segments of a member are converted into a sort of hand,. and in this case the penul timate segment sometimes includes a mus cular mass which may surpass in power the same system in the whole of the limb besides.
Those muscles that put an extremity generally into motion, are attached to the sides of the thoracic cavity, and the apodemata supply them with surfaces of insertion of great extent and very favourably situated as regards their action. They occupy the double rank of cells formed by these lamina2; but they vary too much in their mode of arrangement to admit of our saying any thing general upon this head. The motions of translation, or from n!ace to place, the only kind upon which it seems neces sary to say anything here, are effected in two modes, either by the alternate flexion and ex tension of the trunk, or by the play of the limbs.
In those Crustacea which are formed essen tially for swiinming, the posterior part of the body is the principal agent in enabling the animal to change its place ; but here the mo tions, instead of being lateral, are vertical ; and instead of causing the creature to ad vance they cause it to recede : it is by bend ing the abdomen suddenly downwards, and bringing it immediately under the sternum, that it strikes the water, and consequently by darting backwards that the animal makes its way through that liquid. From what has now been said it may be imagined that the Crustacea whose conformation is the best adapted for swimming, have the abdomen relatively largely developed, and this is, in fact, what we always observe ; the Amphipoda and Decapoda ma croura are examples ; whilst, in the walking Crustacea, such as the Crabs, the Caprella, the Oniscus, &c. this portion of the body attains but very insignificant dimensions.
In the swimming Crustacea the appendages of the penultimate segment of the abdomen also become important organs of locomotion, inasmuch as they for the most part terminate in two broad horizontal plates, which, with the last segment, also become lamelliform, con stitute an extensive caudal fin arranged in the manner of a fan.
We have already said that the thoracic ex tremities alone constitute true ambulatory limbs. When destined for swimming only, their segments are lamelliform, and the palp, as well as the stem, contributes to form the kind of oar which each of them then con stitutes. The Copepoda supply us with in stances of thoracic extremities particularly destined for swimming, and a corresponding structure is observed in certain Podophthalmia, such as the Mysis. (See fig. 386.) To conclude, this stemmatous portion of the thoracic extremities, whilst it still preserves the general form which we have assigned it, is modified in some cases to serve for walking as well as swimming, or to aid the animal as an instrument for burrowing with facility, and making a cavity for shelter among the sand. Thus in the Decapods that burrow, the last seg ment of the tarsus assumes a lanceolated form, and in the swimming lirachyura, the same segment, especially of the last pair of extre mities, appears entirely lamellar.