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.