[11][12] Up to three thoracic segments may be fused with the head to form a cephalothorax; the associated appendages turn forward and are modified as maxillipeds (accessory mouthparts).

Each endopod consist of seven articulating segments; the coxa, basis, ischium, merus, carpus, propodus and dactylus.

In most taxa, each abdominal segment except the last carries a pair of biramous pleopods used for swimming, burrowing, gas exchange, creating a current or brooding eggs.

[13] The digestive tract is straight and the foregut consists of a short oesophagus and a two-chambered stomach, the first part of which contains a gizzard-like "gastric mill" for grinding food.

The fine particles and soluble material are then moved into the midgut where chemical processing and absorption takes place in one or more pairs of large digestive caeca.

The hindgut is concerned with water reclamation and the formation of faeces and the anus is situated at the base of the telson.

The naupliar eye is a characteristic of the nauplius larva and consists of four cup-shaped ocelli facing in different directions and able to distinguish between light and darkness.

[18] They are abundant in all marine ecosystems, and most species are scavengers, although some, such as the porcelain crabs, are filter feeders, and some, such as mantis shrimps, are carnivores.

Female choice may increase the fitness of progeny by avoiding inbreeding that can lead to expression of homozygous deleterious recessive mutations.

Some authors advocate placing Phyllocarida in Phyllopoda, a group used in former classification systems, which would then include branchiopods, cephalocarids and leptostracans.

A molecular study by American biologists Trisha Spears and Lawrence Abele concluded that phylogenetic evidence did not support the monophyly of this grouping, and that Phyllocarida should be regarded as a subclass of Malacostraca that had diverged from the main lineage at an early date.

They are found worldwide from the intertidal zone to the deep ocean, all but one species being benthic (living on the seabed).

They have a dorso-ventrally flattened body and a shield-like carapace and are armed with powerful, raptorial claws normally carried in a folded position.

They are benthic, mostly hiding in cracks and crevices or living in burrows, some emerging to forage while others are ambush predators.

[7][11] The Eumalocostraca contains the vast majority of the approximately 40,000 living species of malacostracans and consists of three superorders, Syncarida, Peracarida and Eucarida.

They are found in marine, freshwater and terrestrial habitats and include Amphipoda, Cumacea, Isopoda and Mysida.