Thus we see that the crayfish has a jointed and segmented body, the rings of which it is composed being very obvious in the abdomen, but more obscurely traceable elsewhere; that it has no fewer than twenty pairs of what may be called by the general name of _appendages_; and that these appendages are turned to different uses, or are organs of different functions, in different parts of the body. The crayfish is obviously a very complicated piece of living machinery. But we have not yet come to the end of all the organs that may be discovered even by cursory inspection. Every one who has eaten a boiled crayfish, or a lobster, knows that the great shield, or carapace, is very easily separated from the thorax and abdomen, the head and the limbs which belong to that region coming away with the carapace. The reason of this is not far to seek. The lower edges of that part of the carapace which belongs to the thorax approach the bases of the legs pretty closely, but a cleft-like space is left; and this cleft extends forwards to the sides of the region of the mouth, and backwards and upwards, between the hinder margin of the carapace and the sides of the first ring of the abdomen, which are partly overlapped by, and partly overlap, that margin. If the blade of a pair of scissors is {25} carefully introduced into the cleft from behind, as high up as it will go without tearing anything, and a cut is then made, parallel with the middle line, as far as the cervical groove, and thence following the cervical groove to the base of the outer foot-jaws, a large flap will be removed. This flap of the carapace is called the _branchiostegite_ (fig. 1, _bg_), because it covers the gills or _branchiæ_ (fig. 4), which are now exposed. They have the appearance of a number of delicate plumes, which take a direction from the bases of the legs upwards and forwards behind, upwards and backwards in front, their summits converging towards the upper end of the cavity in which they are placed, and which is called the _branchial chamber_. These branchiæ are the respiratory organs; and they perform the same functions as the gills of a fish, to which they present some similarity. If the gills are cleared away, it is seen that the branchial cavity is bounded, on the inner side, by a sloping wall, formed by a delicate, but more or less calcified layer of the exoskeleton, which constitutes the proper outer wall of the thorax. At the upper limit of the branchial cavity, the layer of exoskeleton is very thin, and turning outwards, is continued into the inner wall or lining of the branchiostegite, which is also very thin (_see_ fig. 15, p. 70).