The crab class, or Crustacea, have, like the insects, centipedes, spiders, and scorpions, a body built up of successive rings or segments. The earth-worms (as every one knows) and marine bristle-bearing worms also show this feature in the simplest and most obvious way. The vertebrates, with their series of vertebræ or backbone-pieces and the body muscles attached ring-wise to them, show the same condition. The marine worms have a soft skin and a pair of soft paddle-like legs upon each ring of the body, often to the number of a hundred such pairs. But the crab class and the classes called insects, centipedes, arachnids, and millipedes are remarkable for the hard, firm skin, or "cuticle," which is formed on the surface of their bodies and of their legs, which, as in the marine worms, are present--a pair to each body-ring or segment--often along the whole length of the body as in centipedes. This hard cuticle is impregnated with lime in the bigger members of the crab class, such as the lobster. It is not equally thick and hard all over the surface of the lobster, but is separated by narrow bands of thin, soft cuticle into a number of harder pieces, thus rendered capable of being bent or "flexed" on one another. Thus the body is jointed into a series of rings, and the legs are also divided each into several joints (as many as seven), which gives them flexibility and so usefulness of various kinds. The various joints are "worked" by powerful muscles, which are fixed internally to the cuticle and pass from one hard ring or segment, whether of body or of leg, to a neighbouring ring.