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. [Illustration: FIG. 205.--APPENDAGES OF A LOBSTER 1. Second maxilla. 2. Third foot-jaw. 3. Third walking leg. 4. Fifth walking leg] Lastly, beneath the abdomen, are paired limbs called _swimmerets_, which are used as paddles, and probably assist the animal more or less in its progress through the water; but the principal organ of locomotion in the _macrura_ is undoubtedly the powerful muscular abdomen, aided by the broad and fanlike tail formed by the appendages of the last segment. To demonstrate this fact, put a live lobster, or even a shrimp, in a still rock pool, and threaten it from before, when it will rapidly retreat backwards by a series of powerful jerks, produced by suddenly doubling its abdomen forwards beneath its body. In addition to the external characters above mentioned, there are many interesting features connected with the internal structure of the lobster that may be studied on making easy dissections. Thus, the gills, which are attached to the bases of the thoracic limbs, may be exposed by cutting away the side of the carapace, and at the same time we may discover the bailing organ by means of which a current of water is kept flowing forwards through the gill-cavity to keep up the necessary supply of oxygen for respiration. The removal of the upper portion of the carapace will expose the heart and some of the principal bloodvessels, and also the stomach with its powerful and complicated ‘gastric mill,’ formed by the hardening of portions of the wall of the latter organ for the purpose of crushing and masticating the food. Then, if these organs be carefully removed from above, together with the others we have not space to describe, and the powerful muscles that fill up the segments of the abdomen, the chain of ganglia and their connecting nerve cords that form the central part of the nervous system may be seen extending along the central portion of the body. [Illustration: FIG. 206.--LONGITUDINAL SECTION OF THE LOBSTER _a_, antenna; _r_, rostrum or beak; _o_, eye; _m_, mouth; _s_, stomach; _in_, intestine; _l_, liver; _gl_, gills; _h_, heart; _g_, genital organ; _ar_, artery; _n_, nerve ganglia] Several species of lobsters inhabit our seas, but they are generally to be found beyond the tide-marks, and are, therefore, not often caught by sea-side collectors without the aid of some kind of trap or the assistance of fishermen. The common lobster (_Homarus vulgaris_), however, is often left behind by the receding tide on our rocky coasts, and may be seen and caught if one knows where to look and how to capture.