The Arthropods differ among themselves in the number of body-rings, the enlargement or dwindling of certain rings, and the fusion of a larger or smaller number of the rings to form a composite head, or a jointless mid-body or hind-body. The successive legs are primarily and essentially like to one another, and each body-ring, with its pair of legs, is but a repetition of its fellows. At the same time, in the different classes included as "Arthropoda" a good deal of difference has been attained in the structure of the legs, and they have in each class a different form and character in successive regions of the body, distinctive of the class, and are sometimes, but not always, absent from many of the hinder rings. All these Arthropods agree in having a leg on each side immediately behind the mouth--belonging to a body-ring, which is fused with others to form the head--very specially shortened, of great strength and firmness, and shaped so as to be pulled by a powerful muscle attached to it, against its fellow of the opposite side, which is similarly pulled. These two stumpy legs form thus a powerful pair of nippers called "the mandibles." They are jaws, although they were in the ancestors of the Arthropods merely legs. These jaw-legs, or leg-jaws, are characteristic of all the crab class, as well as of the other Arthropods, but no bristle-worm or other animal has them. The jaws of marine worms are of a totally different nature. So are the jaws of snails, whelks, and cuttle-fish. Many of the crab class have not one only, but several, pairs of legs following the mouth converted into jaws. Thus, if you examine a big shore-crab, or, better, an edible crab, and a lobster, and a large prawn, you will find that they all have five pairs of legs converted into short foliaceous jaws (hence called "foot-jaws"), and overlying the first very strong pair, or mandibles. Following these "foot-jaws" you find in a crab or a lobster the great nipping claws and the four large walking legs--the same in proportion and shape in crab, lobster, and prawn, much bigger than the foot-jaws. But the curious thing is that if you set them out and carefully compare them (for they are not simple jointed limbs, but each has two or even three diverging stems carried on a basal joint), you will find a strange and fascinating "likeness in unlikeness," or an agreement of the parts of which they are built, and yet a difference between all of them. The rings of the body to which the jaw-legs and legs are attached are fused into one unjointed piece. The spine in front of the mouth and the support of the eyes and the feelers or "antennæ" are fused with that piece. It forms on the back a great shield--often called "the head"--which overhangs and is bent down over the sides of this region, so as to protect the gills, which you can see by cutting away the overhanging flap.