The antennæ ([Fig. 5], C) can be shown, without difficulty, to conform to the same plan of structure as the other appendages. The two segments of the protopodite are short, but distinct; the endopodite forms the long lash, or flagellum, composed of very numerous small segments; the exopodite is reduced to a small movable scale or spine.
The antennules ([Fig. 5], B) seem at first sight to present the two-branched type of structure in its simplest form; but there is considerable doubt as to whether the two lashes which each bears on a three-segmented stalk are really equivalent to the endopodite and exopodite.
The movable stalks which carry the eyes ([Fig. 5], A) have been considered by some to belong to the series of the appendages, and to be, in fact, modified limbs. If this be the case, we have here the greatest simplification which the limb undergoes in the Lobster, for each eye-stalk consists only of two segments: the first small and incompletely formed, the second in the form of a short cylinder, having the eye at its end. There are, however, reasons for doubting whether the eye-stalks are really appendages.
The hard outer covering of the Lobster not only protects and gives support to the internal organs, but also affords points of attachment for the muscles by means of which the animal moves. In other words, it plays the part of a skeleton; but since, unlike the skeleton of vertebrate animals, it is outside instead of inside the soft parts of the body, it is known as an exoskeleton. Closer examination shows that this outer covering is really continuous over the whole of the body and limbs, but is thin and soft at the joints, allowing the parts to move one upon another. It is composed of a horn-like substance known as chitin, which, except at the joints, is hardened by the deposition in it of carbonate and other salts of lime.
As this external covering does not increase in size after it has been formed, and as it cannot stretch to any great extent, the Lobster requires to cast its shell at intervals as it grows. In this process of moulting the integument of the back splits between the carapace and the first abdominal somite. The body and limbs are gradually worked loose and withdrawn through the opening, leaving the cast shell with all its appendages almost entire. The new covering, which had been formed underneath the old before moulting, is at first quite soft, and the animal rapidly increases in size owing to the absorption of water. The shell then gradually hardens by the deposition of lime salts.
Fig. 6—Dissection of Male Lobster, from the Side. (From British Museum Guide.)
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The internal anatomy ([Fig. 6]) presents many points of interest which can only be briefly touched on here. The food-canal consists of a short gullet leading into a capacious stomach, from which the straight intestine runs to the vent on the under-side of the telson. The stomach has a most remarkable and complicated structure. It consists of two chambers, a larger in front and a smaller behind, which are lined by a continuation of the chitinous outside covering of the body. This chitinous lining is thickened in places to form a system of plates and levers connected with three strong teeth set in the narrow opening between the two chambers. By the action of muscles attached to certain of these plates the teeth work together so as to divide up the food more finely than had been done by the mandibles and other jaws. The whole apparatus, in fact, serves as a kind of gizzard, and is known as the gastric mill.