Every one knows the structure of a lobster's tail and of its legs, which can be readily examined in illustration of my statement, and the same structure can be seen in the leg of a beetle or a fly. Naturalists term all this series of creatures with hard-jointed cuticle, to which the muscles are attached, including the crab class, the insects, centipedes, spiders, and scorpions, "jointed-leg owners," or Arthropods. It is easy to appreciate this characteristic difference which separates the Arthropods from other animals. The sea-worms differ from them, in that they have soft cuticle, but stiffen and render their paddle-like legs firm by squeezing the liquid of the body into them in the same sort of way as the sea-anemones distend their tentacles with liquid, though in that case the liquid is sea-water taken in by the mouth. The Molluscs also distend their muscular lobe, or "foot" as it is called, by pressing the blood from the rest of the body into it, and so making it swell and become stiff, so that the muscles can work it; when not distended in that way it is flaccid. The Vertebrates (bony animals) and the star-fishes have again another and peculiar mechanism. Their muscles are attached to hard internal pieces, sometimes cartilaginous but often calcareous or bony, which are spoken of as "the internal skeleton." There are thus three distinct kinds of mechanism in animals for giving the necessary resisting surfaces, hinged or jointed to one another, and made to "play" one on the other by the alternate contraction and relaxation of the muscles attached to them.