The Crayfish: An Introduction to the Study of Zoology.

by Thomas Henry Huxley

One example of this has already been given in the curious mechanism of the gastric mill. Another may be found in the chela which terminates the forceps. If the {94} articulation of the last joint (fig. [20], dp) with the one which precedes it (prp) is examined, it will be found that the base of the terminal segment (dp) turns on two hinges (x), formed by the hard exoskeleton and situated at opposite points of the diameter of the base, on the penultimate segment; and these hinges are so disposed that the last joint can be moved only in one plane, to or from the produced angle of the penultimate segment (prp), which forms the fixed claw of the chela. Between the hinges, on both the inner and the outer sides of the articulation, the exoskeleton is soft and flexible, and allows the terminal segment to play easily through a certain arc. It is by this arrangement that the direction and the extent of the motion of the free claw of the chela are determined. The source of the motion lies in the muscles which occupy the interior of the enlarged penultimate segment of the limb. Two muscles, one of very great size (m), the other smaller (m′), are fastened by one end to the exoskeleton of this segment. The fibres of the larger muscle converge to be fixed into the two sides of a long flat process of the chitinous cuticula, on the inner side of the base of the terminal segment, which serves as a tendon (t); while those of the smaller muscle are similarly attached to a like process which proceeds from the outer side of the base of the terminal segment (t′). It is obvious that, when the latter muscle shortens it must move the apex of the terminal segment (dp) away from the end of the fixed claw; while, {95} when the former contracts, the end of the terminal segment is brought towards that of the fixed claw.

A living crayfish is able to perform very varied movements with its pincers. When it swims backwards, these limbs are stretched straight out, parallel with one another, in front of the head; when it walks, they are usually carried like arms bent at the elbow, the “forearm” partly resting on the ground; on being irritated, the crayfish sweeps the pincers round in any direction to grasp the offending body; when prey is seized, it is at once conveyed, with a circular motion, towards the region of the mouth. Nevertheless, these very varied actions are all brought about by a combination of simple flexions and extensions, each of which is effected in the exact order, and to the exact extent, needful to bring the chela into the position required.

The skeleton of the stem of the limb which bears the chela is, in fact, divided into four moveable segments; and each of these is articulated with the segments on each side of it by a hinge of just the same character as that which connects the moveable claw of the chela with the penultimate segment, while the basal segment is similarly articulated with the thorax.

If the axes of all these articulations[7] were parallel, it is obvious that, though the limb might be moved as a whole through a considerable arc, and might be bent in various {96} degrees, yet all its movements would be limited to one plane. But, in fact, the axes of the successive articulations are nearly at right angles to one another; so that, if the segments are successively either extended or flexed, the chela describes a very complicated curve; and by varying the extent of flexion or extension of each segment, this curve is susceptible of endless variation. It would probably puzzle a good mathematician to say exactly what position should be given to each segment, in order to bring the chela from any given position into any other; but if a lively crayfish is incautiously seized, the experimenter will find, to his cost, that the animal solves the problem both rapidly and accurately.

[7] By axis of the articulation is meant a line drawn through the pair of hinges which constitute it.

The mechanism by which the retrograde swimming of the crayfish is effected, is no less easily analysed. The apparatus of motion is, as we have seen, the abdomen, with its terminal five-pointed flapper. The rings of the abdomen are articulated together by joints (fig. [21], ×) situated a little below the middle of the height of the rings, at opposite ends of transverse lines, at right angles to the long axis of the abdomen.

Each ring consists of a dorsal, arched portion, called the tergum (fig. [21]; fig. [36], p. 142, t. XIX), and a nearly flat ventral portion, which is the sternum (fig. [36], st. XIX). Where these two join, a broad plate is sent down on each side, which overlaps the bases of the abdominal appendages, and is known as the pleuron (fig. [36], pl. XIX). {97} The sterna are all very narrow, and are connected together by wide spaces of flexible exoskeleton.

FIG. 21.—Astacus fluviatilis.—Two of the abdominal somites, in vertical section, seen from the inner side, to show ×, ×, the hinges by which they are articulated with one another (× 3). The anterior of the two somites is that to the right of the figure.

When the abdomen is made straight, it will be found that these intersternal membranes are stretched as far as they will yield. On the other hand, when the abdomen is bent up as far as it will go, the sterna come close together, and the intersternal membranes are folded.