The /Decapoda/ are named from their ten walking-legs. The higher forms of crustaceans belong to this order. All the /Decapoda/ have a similar anatomy, but are placed in two subdivisions [pg258] according to their external form. In /Macrura/, the first subdivision, belong the lobsters, crawfish, shrimps, prawns, and hermit-crabs, animals having a long and more or less cylindrical body, with the abdomen extended; in /Brachyura/, the second subdivision, are placed the crabs, animals having the thorax broad and flat, and the abdomen bent under the thorax. The /Decapoda/ have twenty segments, all of which, except the last one, have, at some period of life, a pair of appendages. The first two pairs of appendages, or, in the stalk-eyed forms, the first three pairs, are especially connected with the senses, and are often fringed with hairs, which are also considered to have a sense-function. The /antennules/, or first pair of appendages after the eye-stalks, are sometimes divided into two or three branches. At the base of the antennules are the ears. The /antennæ/, or second pair of appendages, are undivided, but are larger than the first pair, and are often very long. At the base of the antennæ are the renal glands. Both the antennules and the antennæ are slender, elongated, movable, and full of joints. In some species they are greatly modified, as in /Scyllarus/, where they are developed into broad swimming-plates and, perhaps, as shovels for burrowing; in some amphipods they are used as swimming-organs. (Plate LX.) [Illustration: PLATE LX. External Anatomy of a Lobster. /C/, carapace; /e/, eye; /g/, gill; /m/, metastoma; /n/, endopodite; /p/, epipodite; /x/, exopodite; I-VII, abdominal segments; 1, antennula; 2, antenna; 3, mandibles; 4, 5, maxillæ; 6, 7, 8, maxillipeds; 9, big pincer; 10-13, walking-feet.]