(_p_), the terminal joint of which is oval and beset with numerous strong setæ, which are especially abundant along its anterior edge. [Illustration: 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.] {172} In the antenna (fig. 48, C) the protopodite is two-jointed. The basal segment is small, and its ventral face presents the conical prominence on the posterior aspect of which is the aperture of the duct of the renal gland (_gg_). The terminal segment is larger and is subdivided by deep longitudinal folds, one upon the dorsal and one upon the ventral face, into two moieties which are more or less moveable upon one another. In front and externally it bears the broad flat _squame_ (_exp_) of the antenna, as an exopodite. Internally, the long annulated “feeler” which represents the endopodite, is connected with it by two stout basal segments. [Illustration: FIG. 48.—_Astacus fluviatilis._—A, eye-stalk; B, antennule; C, antenna of the left side (× 3). _a_, spine of the basal joint of the antennule; _c_, corneal surface of the eye; _exp_, exopodite or squame of the antenna; _gg_, aperture of the duct of the green gland.] {173} The antennule (fig. 48, B) has a three-jointed stem and two terminal annulated filaments, the outer of which is thicker and longer than the inner, and lies rather above as well as external to the latter. The peculiar form of the basal segment of the stem of the antennule has already been adverted to (p. 116). It is longer than the other two segments put together, and near the anterior end its sternal edge is produced into a single strong spine (_a_). The stem of the antennule answers to the protopodite of the other limbs, though its division into three joints is unusual; the two terminal annulated filaments represent the endopodite and the exopodite. Finally, the eyestalk (A) has just the same structure as the protopodite of an abdominal limb, having a short basal and a long cylindrical terminal joint. * * * * * From this brief statement of the characters of the appendages, it is clear that, in whatever sense it is allowable to say that the appendages of the abdomen are constructed upon one plan, which is modified in execution by the excess of development of one part over another, or by the suppression of parts, or by the coalescence of one part with another, it is allowable to say that all the appendages are constructed on the same plan, and are modified on similar principles. Given a general type of appendage consisting of a protopodite, bearing a podobranchia, an endopodite and an exopodite, all the actual appendages are readily derivable from that type. {174}