On examining an appendage from that part of the thorax which lies behind the third maxillipede, say, for example, the sixth thoracic limb (the second walking leg) (fig. 46), the two joints of the protopodite and the five joints of the endopodite are at once identifiable, and so is the podobranchia; but the exopodite has vanished altogether. In the eighth, or last, thoracic limb, the podobranchia has also disappeared. The fifth and sixth limbs also differ from the seventh and eighth, in being chelate; that is to say, one angle of the distal end of the propodite is prolonged and forms the fixed leg of the pincer. The produced angle is that which is turned downwards when the limb is fully extended (fig. 46). In the forceps, the great chela is formed in just the same way; the only important difference lies in the fact that, as in the external maxillipede, the basipodite and the ischiopodite are immoveably united. Thus, {170} the limbs of the thorax are all reducible to the same type as those of the abdomen, if we suppose that, in the posterior five pair, the exopodites are suppressed; and that, in all but the last, podobranchiæ are superadded. [Illustration: FIG. 46.—_Astacus fluviatilis._—The second ambulatory leg of the left side (× 3). _cxp_, coxopodite; _bp_, basipodite; _br_, gill; _cxs_, coxopoditic setæ; _e_, lamina of gill or epipodite; _ip_, ischiopodite; _mp_, meropodite; _cp_, carpopodite; _pp_, propodite; _dp_, dactylopodite.] Turning to the appendages of the head, the second maxilla (fig. 47, C) presents a further modification of the disposition of the parts seen in the first maxillipede. The coxopodite (_cxp_) and the basipodite (_bp_) are still thinner and more lamellar, and are subdivided by deep fissures which extend from their inner edges. The endopodite (_en_) is very small and undivided. In the place of the exopodite and the epipodite there is only one great plate, the scaphognathite (_sg_) which either is such an epipodite as that of the first maxillipede with its anterior basal process much enlarged, or represents both the exopodite and the epipodite. In the first maxilla (B), the exopodite and the epipodite have disappeared, and the endopodite (_en_) is insignificant and unjointed. In the mandibles (A), the representative of the protopodite is strong and transversely elongated. Its broad inner or oral end presents a semicircular masticatory surface divided by a deep longitudinal groove into two toothed ridges. The one of these follows the convex anterior or inferior contour of the masticatory surface, projects far beyond the other, and is provided with a sharp serrated edge; the other (fig. 43, _a_) gives rise to the straight posterior or superior contour of the masticatory surface, and is more obtusely tuberculated. In front, the inner {171} ridge is continued into a process by which the mandible articulates with the epistoma (fig. 47, A, _ar_). The endopodite is represented by the three-jointed _palp_