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. [Illustration: 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.]