Crayfishes, lobsters and the like have groups of plume-like gills (corresponding in the most ancient forms to the number of the legs and jaw-legs) overhung and hidden by the sides of the great shield or "head" of the animal. The common lobsters and crayfishes retain most of these in full size and activity, but have lost in the course of geologic ages the original complete number. These plume-like gills–each half an inch or so in length–are attached, some to the bases of the legs and some to the sides of the body above the legs. In the ancestral form there were thirty-two plumes on each side, twenty-four attached to the bases of the legs, and eight placed each at some distance above the connection of one of the eight legs with the side of the body. It is those on the side of the body which have suffered most diminution in the course of the development of modern crayfishes (and lobsters) from the ancestral form provided with the full equipment of thirty-two gill-plumes on each side. In fact, only one _well-grown_ gill-plume, out of the eight which should exist on each side of the body-wall, is to be found–and that is the one placed above the insertion of the hindermost or eighth of the eight legs (eight when we reckon the three jaw-legs as "legs" as well as the five walking-legs). In front of this the side or wall of the body is bare of gill-plumes though they are present in full size on the basal part of most of the legs. Nevertheless, when one examines carefully with a lens the bare side of the body overhung by the head-shield or "carapace," one finds in a specimen of the common English "pale-footed crayfish" a very minute gill-plume high above the articulation of the seventh leg and another above the articulation of the sixth leg. They are small dwindled things, as though on the way to extinction, and are the mere vestiges of what were once well-grown gill-plumes, and still are so in the rock lobster and some prawns. In the red-footed crayfish of the Continent (Astacus fluviatilis) yet another minute vestige of a gill-plume is found, farther in front, on the body-wall above the fifth leg on each side of the animal. This furnishes a definite mark or character by which we can distinguish the red-footed crayfish from the common English pale-footed one. But these three rudimentary gill-plumes in the red-foot species, and two in the pale-foot species are all that until lately were recorded. The region of the body-wall above the fourth, third, second, and first of the legs was declared to be devoid even of a vestige of the branchial plumes which were there in ancestral forms, and have been retained more or less in some exceptional prawn-like creatures allied to the crayfish.