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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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