![]() ![]() Trilobites, for instance, also possessed biramous appendages. It is unclear whether the biramous condition is a derived state which evolved in crustaceans, or whether the second branch of the limb has been lost in all other groups. Ĭrustacean appendages are typically biramous, meaning they are divided into two parts this includes the second pair of antennae, but not the first, which is usually uniramous, the exception being in the Class Malacostraca where the antennules may be generally biramous or even triramous. The number and variety of appendages in different crustaceans may be partly responsible for the group's success. The abdomen in malacostracans bears pleopods, and ends in a telson, which bears the anus, and is often flanked by uropods to form a tail fan. All other classes of crustaceans have a limbless abdomen, except from a telson and caudal rami which is present in many groups. Malacostraca and Remipedia (and the hexapods) have abdominal appendages. : 289Įach somite, or body segment can bear a pair of appendages: on the segments of the head, these include two pairs of antennae, the mandibles and maxillae the thoracic segments bear legs, which may be specialised as pereiopods (walking legs) and maxillipeds (feeding legs). Various parts of the exoskeleton may be fused together. The shell around each somite can be divided into a dorsal tergum, ventral sternum and a lateral pleuron. The crustacean body is protected by the hard exoskeleton, which must be moulted for the animal to grow. The head and thorax may be fused together to form a cephalothorax, which may be covered by a single large carapace. The body of a crustacean is composed of segments, which are grouped into three regions: the cephalon or head, the pereon or thorax, and the pleon or abdomen. Structure A shed carapace of a lady crab, part of the hard exoskeleton Body structure of a typical crustacean – krill The scientific study of crustaceans is known as carcinology (alternatively, malacostracology, crustaceology or crustalogy), and a scientist who works in carcinology is a carcinologist. ![]() Krill and copepods are not as widely fished, but may be the animals with the greatest biomass on the planet, and form a vital part of the food chain. More than 7.9 million tons of crustaceans per year are harvested by fishery or farming for human consumption, consisting mostly of shrimp and prawns. The group has an extensive fossil record, reaching back to the Cambrian. ![]() Rhizocephala, fish lice, tongue worms) and some are sessile (e.g. woodlice, sandhoppers), some are parasitic (e.g. Most crustaceans are free-living aquatic animals, but some are terrestrial (e.g. They are distinguished from other groups of arthropods, such as insects, myriapods and chelicerates, by the possession of biramous (two-parted) limbs, and by their larval forms, such as the nauplius stage of branchiopods and copepods. Like other arthropods, crustaceans have an exoskeleton, which they moult to grow. The 67,000 described species range in size from Stygotantulus stocki 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 mass of 20 kg (44 lb). The three classes Cephalocarida, Branchiopoda and Remipedia are more closely related to the hexapods than they are to any of the other crustaceans ( oligostracans and multicrustaceans). ![]() It is now well accepted that the hexapods ( insects and entognathans) emerged deep in the Crustacean group, with the completed group referred to as Pancrustacea. The crustacean group can be treated as a subphylum under the clade Mandibulata. Cladistically included but traditionally excluded taxaĬrustaceans belong to the subphylum Crustacea ( / k r ə ˈ s t eɪ ʃ ə/), and form a large, diverse group of arthropods including decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. ![]()
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