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.
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.
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
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.
All the appendages above mentioned are not only attached to the body by movable joints, but are themselves made up of jointed parts, sometimes a considerable number, each of which, like the segments of the body itself, is surrounded by a ring of hardened skin, and connected with those above and below it by a portion of soft and flexible skin.