Writers on descriptive Zoology usually refer to the parts of the maxillipedes under different names from those which are employed here. The protopodite and the endopodite taken together are commonly called the stem of the maxillipede, while the exopodite is the palp, and the metamorphosed podobranchia, the real nature of which is not recognised, is termed the flagellum.
When the comparison of the maxillipedes with the abdominal members, however, had shown the fundamental uniformity of composition of the two, it became desirable to invent a nomenclature of the homologous parts which should be capable of a general application. The names of protopodite, endopodite, exopodite, which I have adopted as the equivalents of the “stem” and the “palp,” were proposed by Milne-Edwards, who at the same time suggested epipodite for the “flagellum.” And the lamellar process of the first maxillipede is now very generally termed an epipodite; while the podobranchiæ, which have exactly the same relations to the following {168} limbs, are spoken of as if they were totally different structures, under the name of branchiæ or gills.
The flagellum or epipodite of the first maxillipede, however, is nothing but the slightly modified stem of a podobranchia, which has lost its branchial filaments; but the term “epipodite” may be conveniently used for podobranchiæ thus modified. Unfortunately, the same term is applied to certain lamelliform portions of the branchiæ of other crustacea, which answer to the laminæ of the crayfishes’ branchiæ; and this ambiguity must be borne in mind, though it is of no great moment.
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.
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 (p), the terminal joint of which is oval and beset with numerous strong setæ, which are especially abundant along its anterior edge.
FIG. 47.—Astacus fluviatilis.—A, mandible; B, first maxilla; C, second maxilla of the left side (× 3). ar, internal, and ar′, external articular process of the mandible; bp, basipodite; cxp, coxopodite; en, endopodite; p, palp of the mandible; sg, scaphognathite; x, internal process of the first maxilla.
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