Approximately 30,000 species make up this Subphylum. Most are aquatic; of these, the majority are marine but some are found in fresh water. Members of the Subphylum include lobsters, crabs, crayfish, shrimp, copepods, barnacles, and several other groups of organisms. All have two pairs of antennae, a pair of mandibles, a pair of compound eyes (usually on stalks), and two pair of maxillae on their heads, followed by a pair of appendages on each body segment (crustacean bodies usually are made up of head, thorax, and abdomen, although the segments composing these tagmata differ among different Classes). The appendages are primitively branched (biramous), and although this condition is modified in many species, adults always have at least some biramous appendages. Crustaceans respire via gills. Like other arthropods, all have a hard but flexible exoskeleton.
Most crustaceans are free-living, but some are sessile and a few are even parasitic. Most use their maxillae and mandibles to take in food. The walking legs, including specialized chelipeds, may be used to help capture prey. Some crustaceans filter tiny plankton or even bacteria from the water; others are active predators; while still others scavenge nutrients from detritus.
Most crustaceans are dioecious. The actual mechanisms by which fertilization is achieved vary greatly. Some crustaceans hatch young that are like miniature adults; others go through a larval stage called a nauplius.
Many species, including lobsters, crayfish, barnacles, and crabs are important to human economies, some very much so. Others, such as krill, are at the base of extremely important marine food chains. Still others are crucial in recycling nutrients trapped in the bodies of dead organisms.
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Hickman, C.P. and L. S. Roberts. 1994. Animal Diversity. Wm. C. Brown, Dubuque, IA.
Brusca, R. C., and G. J. Brusca. Invertebrates. 1990. Sinauer Associates, Sunderland, MA.
Pearse, V., J. Pearse, M. Buchsbaum, and R. Buchsbaum. 1987. Living Invertebrates. Blackwell Scientific Publications, Palo Alto, Ca.
Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor.