Another important point to be noticed is that, in front of the mouth, the sternum of the antennary somite (fig. [43], III) is inclined at an angle of 60° or 70° to the direction of the sterna behind the mouth. The sternum of the antennulary somite (II) is at right angles to the latter; and that of the eyes (I) looks upwards as well as forwards. Hence, the front of the head beneath the rostrum, though it looks forwards, or even upwards, is homologous with the sternal aspect of the other somites. It is for this reason that the feelers and the eyestalks take a direction so different from that of the other appendages. The change of aspect of the sternal surface in front of the mouth, thus effected, is what is termed the cephalic flexure.
Since the skeleton which invests the trunk of the crayfish is made up of a twenty-fold repetition of somites, homologous with those of the abdomen, we may expect to find that the appendages of the thorax and of the head, however unlike they may seem to be to those of the abdomen, are nevertheless reducible to the same fundamental plan. {164}
The third maxillipede is one of the most complete of these appendages, and may be advantageously made the starting point of the study of the whole series.
FIG. 44.—Astacus fluviatilis.—The third or external maxillipede of the left side (× 3). e, lamina, and br, branchial filaments of the podobranchia; cxp, coxopodite; cxs, coxopoditic setæ; bp, basipodite; ex, exopodite; ip, ischiopodite; mp, meropodite; cp, carpopodite; pp, propodite; dp, dactylopodite.
Neglecting details for the moment, it may be said that the appendage consists of a basal portion (fig. [44], cxp, bp), {165} with two terminal divisions (ip to dp, and ex), which are directed forwards, below the mouth, and a third, lateral appendage (e, br), which runs up, beneath the carapace, into the branchial chamber. The latter is the gill, or podobranchia, attached to this limb, and it is something not represented in the abdominal limbs. But, with regard to the rest of the maxillipede, it is obvious that the basal portion (cxp, bp) represents the protopodite, and the two terminal divisions the endopodite and the exopodite respectively. It has been observed that, in the abdominal appendages, the extent to which segmentation occurs in homologous parts varies indefinitely; an endopodite, for example, may be a continuous plate, or may be subdivided into many joints. In the maxillipede, the basal portion is divided into two joints; and, as in the abdominal limb, the first, or that which articulates with the thorax, is termed the coxopodite (cxp), while the second is the basipodite (bp). The stout, leg-like endopodite appears to be the direct continuation of the basipodite; while the much more narrow and slender exopodite articulates with its outer side. The exopodite (ex) is by no means unlike one of the exopodites of the abdominal limbs, consisting as it does of an undivided base and a many-jointed terminal filament. The endopodite, on the contrary, is strong and massive, and is divided into five joints, named, from that nearest to the base onwards, ischiopodite (ip), meropodite (mp), carpopodite (cp), propodite (pp), and dactylopodite (dp). {166}
FIG. 45.—Astacus fluviatilis.—A, the first; B, the second maxillipede of the left side (× 3). cxp, coxopodite; bp, basipodite; e, br, podobranchia; ep, epipodite; en, endopodite; ex, exopodite; ip, ischiopodite; mp, meropodite; cp, carpopodite; pp, propodite; dp, dactylopodite.
The second maxillipede (fig. [45], B) has essentially the same composition as the first, but the exopodite (ex) is relatively larger, the endopodite (ip–dp) smaller and softer; and, while the ischiopodite (ip) is the longest joint in the third maxillipede, it is the meropodite (mp) which is longest in the second. In the first maxillipede (fig. [45], A) a great modification has taken place. The coxopodite (cxp) and the basipodite (bp) are broad thin plates with setose cutting edges, while the endopodite (en) is short and only two-jointed, and the undivided portion of the exopodite (ex) is very long. The place of {167} the podobranchia is taken by a broad soft membranous plate entirely devoid of branchial filaments (ep). Thus, in the series of the thoracic limbs, on passing forwards from the third maxillipede, we find that though the plan of the appendages remains the same; (1) the protopodite increases in relative size; (2) the endopodite diminishes; (3) the exopodite increases; (4) the podobranchia finally takes the form of a broad membranous plate and loses its branchial filaments.