 Crustacean, crustaceans form a large, diverse arthropod taxon which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill, woodlache, and barnacles. The crustacean group is usually treated as a subphylum, and because of recent molecular studies it is now well accepted that the crustacean group is parafiotic, and comprises all animals in the pancrustacea clata than hexapods. Some crustaceans are more closely related to insects and other hexapods than they are to certain other crustaceans. The 67,000-described species range in size from Stygotin to Booze stocky at 0.1 mm, 0.004 in to the Japanese spider crab with a leg span of up to 3.8 m 12.5 ft and a massive 20 kg 44 lb. Like other arthropods, crustaceans have an exoskeleton, which they mold to grow. They are distinguished from other groups of arthropods, such as insects, myriapods and chelacerates, by the possession of birame-mouse-to-parted limbs, and by their larval forms, such as the norply-used stage of brachyapods and cotal-pods. Most crustaceans are free-living aquatic animals, but some are terrestrial e.g. woodlachacean or parasitic e.g. rhizosephala, fish-lice, tongue worms and some are sephal e.g. barnacles. The group has an extensive fossil record, reaching back to the Cambrian, and includes living fossils such as triops cancriformis, which has existed apparently unchanged since the Triassic period. More than 10 million tons of crustaceans are produced by fishery or farming for human consumption, the majority of it being shrimp and prawns. Crill and cotal-pods are not as widely fish, but may be the animals with the greatest biomass on the planet, and form a vital part of the food chain. The scientific study of crustaceans is known as carcinology alternatively, malacostracotlogy, crustacea-logy or crustalogy and a scientist who works in carcinology is a carcinologist.