In the _Macrura_, as with other divisions of the crustaceans, we meet with very interesting modifications of the appendages, adapted to quite a variety of uses; and if the reader is unacquainted with these adaptations of structure to habit he cannot do better than secure a lobster or crayfish for study. It will be observed that the body may be divided into two main portions--the _cephalothorax_, consisting of head and thorax combined, and the _abdomen_. The former is composed of fourteen segments, so thoroughly fused together that they are denoted only by the fourteen pairs of appendages to which they give attachment, while the calcified skin forms one continuous shield surrounding the whole. The abdomen, on the other hand, consists of six distinct segments, each of which is surrounded by its own ring of the hardened integument, and is connected with its neighbours by means of a portion of uncalcified skin that renders the whole very flexible. A groove in the front portion of the great shield (_carapace_) marks the division between the head and the thorax, the former composed of six, and the latter of eight united segments. [Illustration: FIG. 203.--PARTS OF LOBSTER’S SHELL, SEPARATED, AND VIEWED FROM ABOVE] The calcareous covering of each segment consists of an upper portion, called the _tergum_, and a lower, named the _sternum_, united at the sides; the sternal portion of the cephalothorax, which gives attachment to the walking limbs, is a most complicated and beautifully formed structure. [Illustration: FIG. 204.--A SEGMENT OF THE ABDOMEN OF A LOBSTER _t_, tergum; _s_, sternum, bearing a pair of swimmerets; _h_, bloodvessel; _d_, digestive tube; _n_, nerve chain] The six pairs of appendages belonging to the head are easily made out with a little care. The first are the jointed _eye-stalks_ that bear the compound eyes previously described; and these are followed by two pairs of _antennæ_, or feelers, the first being shorter and double, while the second are very long. The former contain the organs of hearing. Then, in front of the mouth, and completely hiding it, are a pair of strong _mandibles_ or jaws that move horizontally, and the two pairs of _maxillæ_ that are also employed in reducing the food. Following these, but belonging to the thorax, are three pairs of appendages that are known as foot-jaws; for, although they assist the preceding organs in breaking up the food, they bear a resemblance in some respects to the longer limbs behind them. Of the latter there are five pairs (hence the term _decapoda_), the first being a very powerful pair of seizers or pincers, and the remaining four, which are well adapted for walking, terminating in either double or single claws.