The marority of Crustacea are marine forms, the most notable of which are shrimp lobsters crabs and barnacles. Some crustacea including mostly crayfish, live in freshwater habitats. Still others, such as sand fleas, land crabs, and sow bugs live in moist terrestrial environments. The shell of crustaecas vary in thickness and size among the order. It must be periodically molted because the hard structure prohibits growth.
Respiratory organs can also be the gills formed by modifying parts of the appendages.
The nervous system consists of a brain that is connected to the eyes by nerves. The sexes are separate in most crustacea. Sex determining genes probably control the formation of androgenic glands that produce hormones resulting in the development of the male. The males have prehensile appendages for clasping the female while mating. The majority of crustaceas hatch from eggs, which then pass through a series of free swimming larva stages.