limbs, and some of these are modified to serve as jaws. The chief divisions or classes of the Arthropoda are—(i.) Insecta, including Butterflies, Moths, Bees, Wasps, Flies, Beetles, and the like; (ii.) Chilopoda, or Centipedes; (iii.) Diplopoda, or Millipedes[1]; (iv.) Onychophora, including the curious worm-like Peripatus; (v.) Arachnida, or Scorpions, Spiders, Mites, and their allies; and (vi.) Crustacea.

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.

In pursuance of this plan of study, the next chapter will be devoted to a description of the Common Lobster as a type of the Crustacea. In the third chapter a survey of the classification of the group will be given; since, however, the characters on which the classification is based cannot be explained fully without entering into technical details which are beyond the scope of this work, this survey will be restricted to what is necessary for comprehension of the succeeding chapters. In the fourth chapter some account is given of the young or larval stages of Crustacea, and of the changes they undergo in the course of development.

In the next five chapters the Crustacea are classified according to their habitats, and those living in the shallow waters, the depths, and the surface of the ocean, in the fresh waters, and on land, are discussed in turn; while a separate chapter is devoted to the curious forms that live as parasites on, or as associates with, other animals. The last two chapters deal respectively with the Crustacea as they affect man, and with the past history of the group as revealed by fossil remains.

CHAPTER II

THE LOBSTER AS A TYPE OF CRUSTACEA

The most noticeable feature distinguishing the Lobster[2] (Fig. 1) at first sight from other familiar animals is the jointed shelly armour that encases its body and limbs. Over the fore part of the body this armour is continuous, forming a shield, or carapace, which projects in front, between the eyes, as a toothed beak, or rostrum; on the hinder part—the tail, or abdomen—it is divided into six segments, or somites, connected with each other by movable joints. Each of these somites carries on the under-side a pair of fin-like limbs, or swimmerets, the last pair of which (uropods) are much larger than the others, and are spread out at the sides of a middle tail-plate, or telson, forming what is known as the tail-fan. Since the fore part of the body also has a series of paired limbs, constructed, as will be shown later, on the same plan as the swimmerets, it is concluded that this part also is built up of somites, which have become soldered together. That this conclusion is correct is shown by comparison with some of the lower Crustacea in which this part of the body is divided up into eight separate somites, like those of the abdomen, each carrying, in place of the swimmerets, a pair of walking legs. In front of these eight somites, forming what is called the thorax, is the head—a part of the body which is never, in any Crustacean, broken up into distinct somites, but which, since it carries five pairs of appendages, must consist of at least five somites. The part of the body covered by the Lobster's carapace includes both the head and the thorax, and is known, therefore, as the cephalothorax. It is necessary to bear in mind that the parts of the body to which the names head, thorax, and abdomen, are applied in Crustacea are by no means exactly equivalent to those which bear the same names in Insects, for example, and that, beyond a rough similarity in position, they have no sort of relation to the parts so named in the body of a vertebrate animal.