It will be best to take an example from about the middle of the scale, where the two elements, viz., essential identity and adaptive modification, are somewhat evenly balanced, and both traceable with ease and certainty. Take, then, a cray-fish, a lobster, or a shrimp. This animal (Fig. 27) has twenty or twenty-one rings and pairs of jointed appendages. The rings are some of them diminished, some of them increased in size. Sometimes several are consolidated; sometimes several are partially or wholly aborted. The appendages are modified in shape and size, according to their position, so as to make them swimming-appendages (swimmerets), walking-appendages (legs), eating-appendages (jaws), and sense-appendages (antennæ). For example, in the abdominal region, or so-called tail, we have seven segments, all being perfect movable rings, each with its pair of jointed appendages, except the last, or _telson_. The appendages of the first ring (Fig. 28, B) are specially modified in the male as organs of copulation (B′). The next four pairs are modified for swimmerets (D′) and for use as holders of the eggs in the female. The appendages of the sixth ring (G) are broad and paddle-shaped, and, together with the telson or seventh ring (H), form the powerful terminal swimmer. Going, now, to the cephalo-thorax: in this either a large number of segments (thirteen or fourteen) are consolidated above to form the upper shell or carapace; or else, as is more probable, two or three of the anterior segments have enlarged and grown backward over, and at the expense of the others, to form this shell. At any rate, it is certain that the carapace is formed of the dorsal portions of a number of segments consolidated together. Below, however, the segments are all distinct, and have each its own pair of appendages. For example, going forward in this region, the five next pairs of appendages are greatly enlarged and very strong, and serve the purpose of locomotion. They are _walking-appendages_. The next two or three pairs are smaller and somewhat modified, but not so much as to obscure their essential similarity to legs. Like legs, they are many-jointed, and like legs, too, they have gills attached to them. They are called maxillipeds, or jaw-feet. They are used like hands to gather food and carry it to the mouth. They are _gathering-appendages_. Then follow three or four pairs still more modified, and used for mastication. They are called maxillæ and mandibles. They are _eating-appendages_. Then follow two pairs, long, many-jointed, with the same kind of curious hinge-joints, which we have in the legs, undoubtedly homologous with all the others, but used for an entirely different purpose, and specially modified for that purpose. They are the antennæ. They are delicate organs of touch and of hearing, for the ear is situated in the basal joint of the anterior pair. Last of all, there is still another pair, jointed and movable, on the ends of which are situated the eyes.