221 



CRUSTACEA. 



CRUSTACEA. 



223 



At the period of Ecdysis, rounded flattened calcareous concretions 

 (carbonate of lime), commonly called Oculi Cuncrorum, are formed 

 at the sides of the stomach of the common river crawfish. (Prep. 

 406, Mus. Coll. of Surg.) 



Every one has occasionally been struck with the difference of size 

 in the members of crabs and lobsters. One claw of these, and other 

 crustaceans which have the claws, when perfect, nearly equal, is often 

 found of its full volume, while the other is comparatively diminutive ; 

 for the animal, upon the limb receiving any injury, has the power of 

 suddenly throwing it off, and the effort does not appear to be attended 

 with pain, though it is frequently made when the system receives a 

 severe shock. [ASTACUS.] The point at which the separation takes 

 place is always in the second articulation, near the basis of the 

 limb, and from the stump, which speedily cicatrises, a new claw buds 

 forth with all the proper articulations, and with an entire though 

 miniature resemblance to the rejected member. This new claw is 

 formed within the old shell and lies folded up until the exuvia are 

 hed, when it appears as a part of the new skeleton. If one of the 

 limbs be severed in any other place than the usual point of separation 

 the stump goes on bleeding, nor does it heal. In such cases the 

 renovating process does not commence until the animal succeeds in 

 separating the remains of the member at the proper point, and this 

 it does by a violent muscular contraction. Some years ago there 

 were some Land-Crabs (Gecarcinus) at the Garden of the Zoological 

 Society, and the apparent ease with which they parted with their 

 smaller legs in order to escape from any one who injudiciously took 

 them up by those members was very remarkable. They did not seem 

 to regard the loss at all, and ran away on the remainder of their legs 

 as if nothing had happened. Mr. Harry Goodsir has pointed out 

 that this power of renewing the members in the Crustacea depends 

 on a small glandlike body seated at the base of each limb. This 

 body consists of a great number of large nucleated cells, which are 

 interspersed throughout a fibro-gelatinous mass. It is supplied by a 

 vessel and a nerve. Mr. Spence Bate describes the development of 

 the shell as follows : 



" Immediately above the heart, a pulp consisting of nucleated cells, 

 areolar tissue, (and blood vessels ?), is formed, extending to the 

 internal surface of the shell, from which it is separated by a layer of 

 pigment which gives colour to the new formation. Towards the 

 base, that in, immediately above the heart, the cells are uniformly large 

 and distinct, while an areolar tissue ramifies throughout the whole. 

 As advance is made from the base, cells of less size mix with them, 

 which increase in number as they diminish, in diameter, until they 

 approach the layer of pigment, immediately beneath which they 

 adapt themselves by mutual pressure into a polygonal form. The 

 pulp extends over the whole periphery of the crab, immediately 

 beneath the shell ; the thickness of the pulp decreases with the 

 distance from the centre ; and the larger cells- become fewer in 

 number, the mass being made of the smaller cells which become the 

 secreting organs of the future shell, which process commences pre- 

 viously to and is completed after the removal of the exuviae." (' Annals 

 of Nat. Hist.,' vol. vii.) 



Of the nature of the organs of locomotion developed by the 

 external skeleton, Milne-Edwards has given the best account : 



" 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 animals, is required to be so 

 constructed as not to oppose locomotion ; consequently there exist, 

 either between the different rings of the body or the various con- 

 stituent elements of the limbs, articulations destined to admit of 

 motion to a greater or less extent between these different pieces. 

 The structure of these articulations 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 internal 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 intro- 

 duced a kind of corrective of this disadvantage in the structure of 

 the limbs, by changing the directions of the articular axis, whence 

 ensues the possibility of general motions being performed in every 

 direction. Between the two fixed points two opposed empty spaces 

 arf observed, left by the rings severally, and destined to admit of the 

 occurrence of motions of flexion and extension. The tegumentary 

 membrane which fills it never becomes incrusted or calcareous, but 

 always continues soft and flexible. 



" The tegumentary skeleton 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 required 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 attached immediately. Their struc- 

 ture is simple, and each segment in fact, as has already been said, 

 being contrived to move in one fixed and determinate plane, the 

 muscles which communicate motion to it can constitute no more 

 than two systems antagonists 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 muscles that produce these opposite effects, as might 

 have been concluded, are found implanted into the opposite arms of 

 the lever upon which their energy is extended. 



" 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 iu 

 accordance with the nature of the motion produced, it is upon the 

 superior or dorsal aspect of the segments that the extensor muscles 

 are attached. In the trench the two orders of muscles 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 susceptible depend on the size of 

 the interannular spaces above or below the ginglymoid joints ; 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 articula- 

 tions, 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 

 of power, rapidity, and precision in its motions. Each segment of a 

 limb incloses the muscles destined to move that segment which suc- 

 ceeds it, unless it be too short and weak for this end, in which case 

 the muscles themselves have their origin at some point nearer to the 

 medium 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 com- 

 municate 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 penultimate segment some- 

 times includes a muscular 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 

 laminae, but they vary too much in their mode of arrangement to 

 admit of our saying anything generally upon this head. The motion 

 of translation or from place to place, the only kind upon which it 

 seems necessary to say anything here, is effected iu two modes, 

 either by the alternate flexion and extension of the trunk, or by the 

 play of the limbs. 



" In those Crustacea which are formed essentially for swimming, 

 the posterior part of the body is the principal agent in enabling the 

 animal to change its place ; but here the motions, instead of being 

 lateral, are vertical ; and instead of causing the creature to advance 

 they cause it to recede : it is by bending 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 the liquid. [ASTACUS.] From what 

 has now been said it may be imagined that the Crustacea whose con- 

 formation is the best adapted for swimming have the abdomen largely 

 developed, and this is in fact what we always observe ; the Amp/ii- 

 poda and Decapoda Macroura are examples ; whilst in the walking 

 Crustacea, such as the Crabs, the Caprella, the Oniscus, &c., this por- 

 tion 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, constitute an 

 extensive caudal fin arranged in the manner of a fan. We have 

 already said that the thoracic extremities alone constitute true ambu- 

 latory 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 constitutes. 



