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