The Crayfish: An Introduction to the Study of Zoology.

by Thomas Henry Huxley

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.

In the female, the appendages of the second somite of the abdomen are similar to those of the third, fourth, and fifth somites; but in those of the first somite (fig. [37], B), there is a considerable variation. Sometimes, in fact, the appendages of this somite are altogether wanting; sometimes one is present, and not the other; and sometimes both are found. But, when they exist, these appendages are always small; and the protopodite is followed by only one imperfectly jointed filament, which appears to represent the endopodite of the other appendages.

In the male, the appendages of the first and second somites of the abdomen are not only of relatively large size, but they are widely different from the rest, those of the first somite departing from the general type further than those of the second. In the latter (C, C′) there is a protopodite (cx.p, bp) with the ordinary structure, and it is followed by an endopodite (en.p) and an exopodite {147} (ex.p); but the former is singularly modified. The undivided basal part is large, and is produced on the inner side into a lamella (a), which extends slightly beyond the end of the terminal jointed portion (b). The inner half of this lamella is rolled upon itself, in such a manner as to give rise to a hollow cone, something like an extinguisher (C′, a).

The appendage of the first somite (A) is an unjointed styliform body, which appears to represent the protopodite, together with the basal part and the inner prolongation of the endopodite of the preceding appendage. The terminal half of the appendage is really a broad plate, slightly bifid at the summit, but the sides of the plate are rolled in, in such a manner that the anterior half bends round and partially incloses the posterior half. They thus give rise to a canal, which is open at each end, and only partially closed behind.

These two pairs of curiously modified appendages are ordinarily turned forwards and applied against the sterna of the posterior part of the thorax, in the interval between the bases of the hinder thoracic limbs (see fig. [3], A). They serve as conduits by which the spermatic matter of the male is conveyed from the openings of the ducts of the testes to its destination.