[Illustration: FIG. 37.—_Astacus fluviatilis._—Appendages of the left side of the abdomen (× 3). A, the posterior face of the first appendage of the male; B, the same of the female; C, posterior, and C′, anterior faces of the second appendage of the male; D, the third appendage of the male; E, the same of the female; F, the sixth appendage. _a_, the rolled plate of the endopodite; _b_, the jointed extremity of the same; _bp._, basipodite; _cx.p._, coxopodite; _en.p._, endopodite; _ex.p._, exopodite.] The appendage (fig. 36, _19_), which is attached to an articular cavity situated between the sternum and the epimeron, is seen to consist of a stalk or stem, which is made up of a very short basal joint, the _coxopodite_ (fig. 37, D and E, _cx.p_), followed by a long cylindrical second joint, the _basipodite_ (_b.p_), and receives the name of _protopodite_. At its free end, it bears two flattened narrow {145} plates, of which one is attached to the inner side of the extremity of the protopodite, and is called the _endopodite_ (_en.p_), while the other is fixed a little higher up to the outer side of that extremity, and is the _exopodite_ (_ex.p_). The exopodite is shorter than the endopodite. The endopodite is broad and is undivided for about half its length, from the attached end; the other half is narrower and is divided into a number of small segments, which, however, are not united by definite articulations, but are merely marked off from one another by slight constrictions of the exoskeleton. The exopodite has a similar structure, but its undivided portion is shorter and narrower. The edges of both the exopodite and the endopodite are fringed with long setæ. In the female crayfish, the appendages of this and of the fourth and third somites are larger than in the male (compare _D_ and _E_, fig. 37). The fourth and fifth somites, with their appendages, may be described in the same terms as the third, and in the sixth there is no difficulty in recognising the corresponding parts of the somite; but the appendages (fig. 37, _F_), which constitute the lateral portions of the caudal fin, at first sight appear very different. In their size, no less than in their appearance, they depart widely from the appendages of the preceding somites. Nevertheless, each will be found to consist of a basal stalk, answering to the protopodite (_cx.p_), which however is very broad and thick, and is not divided into two {146} joints; and of two terminal oval plates, which represent the endopodite (_en.p_) and the exopodite (_ex.p_). The latter is divided by a transverse suture into two pieces; and the edge of the larger or basal moiety is beset with short spines, of which two, at the outer end of the series, are larger than the rest. The second somite is longer than the first (fig. 1); it has very broad pleura, while those of the first somite are small and hidden by the overlapping front margins of the pleura of the second somite.