Now the exoskeleton is not of the same constitution throughout these regions. The abdomen, for example, is composed of six complete hard rings (fig. 2, XV–XX), and a terminal flap, on the under side of which the vent (fig. 3, _a_) is situated, and which is called the _telson_ (fig. 2, _t, t′_). All these are freely moveable upon one another, inasmuch as the exoskeleton which connects them is not calcified, but is, for the most part, soft and flexible, like the hard exoskeleton when the lime salts have been removed by acid. The mechanism of the joints will have to be attentively considered by-and-by; it is sufficient, at present, to remark that, wherever a joint, exists, it is produced in the same fashion, by the exoskeleton remaining soft in certain regions of the jointed part. The carapace is not jointed; but a transverse groove is observed about the middle of it, the ends of which run down on the sides and then turn forwards (figs. 1 and 2, _cg_). This is called the _cervical groove_, and it marks off {20} the region of the head, in front, from that of the thorax behind. The thorax seems at first not to be jointed at all; but if its under, or what is better called its _sternal_, surface is examined carefully, it will be found to be divided into as many transverse bands, or segments, as there are pairs of legs (fig. 3); and, moreover, the hindermost of these segments is not firmly united with the rest, but can be moved backwards and forwards through a small space (fig. 3, B; xiv). Attached to the sternal side of every ring of the abdomen of the female there is a pair of limbs, called _swimmerets_. In the five anterior rings, these are small and slender (fig. 3, B; _15, 19_); but those of the sixth ring are very large, and each ends in two broad plates (_20_). These two plates on each side, with the telson in the middle, constitute the flapper of the crayfish, by the aid of which it executes its retrograde swimming movements. The small swimmerets move together with a regular swing, like paddles, and probably aid in propelling the animal forwards. In the breeding female (B), the eggs are attached to them; while, in the male, the two anterior pairs (A; _15, 16_) are converted into the peculiar styles which distinguish that sex.