Читать книгу The Life of Crustacea - W. T. Calman - Страница 8

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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.