It is not easy to summarize in a few words the characters common to all Crustacea, and distinguishing them from the other groups of Arthropoda. As a rough guide to classification, it is useful to remember that an Insect can generally be recognized by having three pairs of walking legs, an Arachnid by having four pairs, and a Centipede or Millipede by having a great many pairs, all nearly alike. The Crustacea, on the other hand, show great diversity in the number and arrangement of their walking or swimming legs, but they rarely show any special resemblance to those of the other large groups of Arthropoda. Thus, for example, a common species of Woodlouse, _Armadillidium vulgare_, is very similar at first sight to the Millipede _Glomeris marginata_, but it has only seven pairs of walking legs, while the Millipede has seventeen or nineteen pairs. More precisely, it may be said of the great majority of Crustacea that they are aquatic animals, breathing by gills or by the general surface of the body, having two pairs of "feelers," or antennæ, on the front part of the head, and at least three pairs of jaws. Exceptions to each of these statements will be mentioned in later chapters in dealing with parasites and other highly modified types. In such cases, however, the larval or young stages afford indications of affinity, and comparison with less modified forms enables us to trace a connection with the typical Crustacea. The best way to form a conception of a group of animals, however, is not to attempt in the first place to define its limits, but to begin by studying the structure of some typical and central species, and afterwards to note the divergences from this type presented by other members of the group. Speaking very generally, it may be said that these divergences are of two kinds. On the one hand there are characters that have no apparent relation to the animal's habits and mode of life, and on the other hand there are modifications of structure which are more or less plainly of use to the animal. It is to characters of the former class that we look for evidence of an animal's affinities, and it is upon them that our systems of classification are chiefly based. The characters of the second class--"adaptive" characters, as they are called--become of importance when we study the animal "as a going concern," so to speak, and endeavour to understand how its life is carried on in relation to its surroundings.