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Crustacean
The crustaceans (Crustacea) are a large group of arthropods (55,000 species), usually treated as a subphylum. They include various familiar animals, such as lobsters, crabs, shrimp and barnacles. They are variously found in marine and freshwater, with a few terrestrial members (such as woodlice.
There are 1,220 barnacle species, 1,000 branchiopod, 13,000 copepod, and 30,000 malacostracan.
Crustaceans typically have a thick carapace covering part of their body. Their appendages are typically biramous, including a second pair of antennae (but not the first).
The classification varies somewhat. The groups listed at right are generally recognized, but several of them are often treated as subclasses of a class Maxillopoda[?]. Evolutionary relationships between the different groups are not entirely clear, making the exact definition of larger groups difficult.
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