It is now well accepted that the hexapods (insects and entognathans) emerged deep in the Crustacean group, with the completed pan-group referred to as Pancrustacea.

[3] 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).

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.

Most crustaceans are free-living aquatic animals, but some are terrestrial (e.g. woodlice, sandhoppers), some are parasitic (e.g. Rhizocephala, fish lice, tongue worms) and some are sessile (e.g. barnacles).

More than 7.9 million tons of crustaceans per year are harvested by fishery or farming for human consumption,[4] consisting mostly of shrimp and prawns.

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.

The body of a crustacean is composed of segments, which are grouped into three regions: the cephalon or head,[5] the pereon or thorax,[6] and the pleon or abdomen.

[17] The main body cavity is an open circulatory system, where blood is pumped into the haemocoel by a heart located near the dorsum.

Many terrestrial crustaceans (such as the Christmas Island red crab) mate seasonally and return to the sea to release the eggs.

[28] Since the opening of the Suez Canal, close to 100 species of crustaceans from the Red Sea and the Indo-Pacific realm have established themselves in the eastern Mediterranean sub-basin, with often significant impact on local ecosystems.

Most decapods carry the eggs attached to the pleopods, while peracarids, notostracans, anostracans, and many isopods form a brood pouch from the carapace and thoracic limbs.

[37] As the larvae mature into adults, the layer migrates to a new position behind the retina where it works as a backscattering mirror that increases the intensity of light passing through the eyes, as seen in many nocturnal animals.

[41] The earliest nomenclatural valid work to use the name "Crustacea" was Morten Thrane Brünnich's Zoologiæ Fundamenta in 1772,[42] although he also included chelicerates in the group.

[44] Although most crustaceans are small, their morphology varies greatly and includes both the largest arthropod in the world – the Japanese spider crab with a leg span of 3.7 metres (12 ft)[45] – and the smallest, the 100-micrometre-long (0.004 in) Stygotantulus stocki.

Studies based on morphology led to the Pancrustacea hypothesis,[47] in which Crustacea and Hexapoda (insects and allies) are sister groups.

More recent studies using DNA sequences suggest that Crustacea is paraphyletic, with the hexapods nested within a larger Pancrustacea clade.

[50] Bowman and Abele (1982) recognised 652 extant families and 38 orders, organised into six classes: Branchiopoda, Remipedia, Cephalocarida, Maxillopoda, Ostracoda, and Malacostraca.

[56][57] The following cladogram shows the updated relationships between the different extant groups of the paraphyletic Crustacea in relation to the class Hexapoda.

Crustaceans have a rich and extensive fossil record, most of the major groups of crustaceans appear in the fossil record before the end of the Cambrian, namely the Branchiopoda, Maxillopoda (including barnacles and tongue worms) and Malacostraca; there is some debate as to whether or not Cambrian animals assigned to Ostracoda are truly ostracods, which would otherwise start in the Ordovician.