In addition to viruses, and plants are animals. Usually they take in organic food which is broken down by digestion. Often they have compact bodies for locomotion. In this category, there are two main subdivisions: Invertebrates (animals without backbones) and Vertebrates (animals with backbones). The former group includes protozoa, single-celled animals which reproduce by fission-simple division; roundworms, which are not segmented, wormlike, with larval stages; flatworms, multicellular animals which reproduce both sexually and asexually; arthropods, which have jointed limbs and a hard outside skeleton celled an exoskeleton, e.g. insects; molluscs, which have a shell protecting a soft body with a large foot e.g. sail.