The Arthropods in their numbers and in their variety of species far exceed all the rest of the animal kingdom put together. Of the animals which live in fresh water a high proportion belong to this phylum. The name ‘Arthropod’ means jointed limbs, the most characteristic feature of the phylum being the fact that the numerous limbs on the body are made up of a number of separate segments. In many Arthropods the body shows some external indication of segmentation like the body of a worm, and to all or some of the segments are attached a pair of limbs. The limbs near the front end of the body have usually a sensory function, as in the antennae or ‘feelers’ of butterflies. The limbs behind the antennae are usually modified into structures to seize and grind up food; these are collectively called the jaws or mouth-parts, of which there may be from three to six pairs. Behind the mouthparts come the limbs used for locomotion (i.e. walking or swimming). The skin on the surface of the animals contains a particular substance called chitin and is waterproof. Frequently most of the skin tends to be rather hard and brittle, and movement would be difficult were it not that between the segments on the body and the leg joints there are thinner, more flexible areas not impregnated with hard substances.