Having learned this much about the origin of the names of the crayfish, we may next proceed to consider those points which an observant Naturalist, who did not {17} care to go far beyond the surface of things, would find to notice in the animal itself. Probably the most conspicuous peculiarity of the crayfish, to any one who is familiar only with the higher animals, is the fact that the hard parts of the body are outside and the soft parts inside; whereas in ourselves, and in the ordinary domestic animals, the hard parts, or bones, which constitute the skeleton, are inside, and the soft parts clothe them. Hence, while our hard framework is said to be an _endoskeleton_, or internal skeleton; that of the crayfish is termed an _exoskeleton_, or external skeleton. It is from the circumstance that the body of the crayfishes is enveloped in this hard crust, that the name of _Crustacea_ is applied to them, along with the crabs, shrimps, and other such animals. Insects, spiders, and centipedes have also a hard exoskeleton, but it is usually not so hard and thick as in the _Crustacea_. If a piece of the crayfish’s skeleton is placed in strong vinegar, abundant bubbles of carbonic acid gas are given off from it, and it rapidly becomes converted into a soft laminated membrane, while the solution will be found to contain lime. In fact the exoskeleton is composed of a peculiar animal matter, so much impregnated with carbonate and phosphate of lime that it becomes dense and hard. [Illustration: FIG. 2.—_Astacus fluviatilis._—Dorsal or tergal views (nat. size). A, male; B, female:—_bcg_, branchio-cardiac groove, which marks the boundary between the pericardial and the branchial cavities; _cg_, cervical groove; these letters are placed on the carapace; _r_, rostrum; _t, t′_, the two divisions of the telson; _1_, eye-stalks; _2_, antennules; _3_, antennæ; _20_, lateral lobes of tail-fin; XV–XX, somites of the abdomen.] It will be observed that the body of the crayfish is naturally marked out into several distinct regions. There {19} is a firm and solid front part, covered by a large continuous shield, which is called the _carapace_; and a jointed hind part, commonly termed the tail (fig. 2). From the perception of a partially real, and partially fanciful, analogy with the regions into which the body is divided in the higher animals, the fore part is termed the _cephalo-thorax_, or head (_cephalon_) and chest (_thorax_) combined, while the hinder part receives the name of _abdomen_.