This taxon, largely composed of free-living creatures, has numerous taxa which are parasitic. The parasitic members are usually ectoparasites, and can be important pests in commercial operations.
Most crustaceans have marine or freshwater life cycles. Most are dioecious. Life cycles are varied, but generally the female produces eggs, which hatch into various larval forms. These undergo various numbers of molts, and transform into other larval forms, which also undergo various numbers of molts. The larval stages act as dispersal forms.
Crustaceans have segmented bodies covered with an exoskeleton. There are eyes, two pairs of antennae, mandibles, and two pairs of maxillae on the head, although these are often highly modified in some of the parasitic groups. The body shows varying degrees of fusion of head, thorax and abdomen, and in some taxa segmentation is not apparent at all. Appendages, when present, are usually biramous. Most female crustaceans brood their eggs in a pair of external egg sacs.