The crustaceans include shrimp, lobsters, crabs, barnacles, isopods and copepods. There are about 47,000 known species, living primarily in marine and freshwater environments. Insects are thought to be a highly successful terrestrial branch of the crustacean lineage. The body plan of most crustaceans includes two tagmata: a cephalothorax (combined head and thorax); and an abdomen. Many crustaceans have a platelike section of exoskeleton called a carapace covering their cephalothorax. Crustaceans are the only arthropods with two pairs of antennae, which they use for sensing the environment. Appendages are generally branched, or biramous (in contrast to insect appendages, which are unbranched). Feeding: crustaceans generally have 4-6 pairs of mouthparts, derived from their jointed appendages. Many crustaceans have claws or other feeding appendages to capture food, as well as mandibles for biting and chewing. Movement: a few crustaceans, such as barnacles, are sessile, but other crustaceans use their appendages for walking, running, and swimming. Reproduction: sexual reproduction, with internal fertilisation, is usual.