CRUSTACEA. 



761 



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. 



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 



VOL. I. 



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 sus- 

 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 laminse; 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 place to 



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