Crustacea

Crustacea (crabs, lobsters, shrimps, slaters, woodlice, barnacles; phylum Arthropoda) Diverse class of arthropods which have two pairs of antennae, one pair of mandibles, and two pairs of maxillae. The limbs are biramous, and are adapted for a wide range of functions. Closely placed setae on the limbs function as filters in filter-feeding species. Respiratory gills are situated on the appendages, but vary greatly in location and number; they are absent only in very small species. In addition to the antennae, sense organs include a pair of compound eyes, and a small, dorsal, median, nauplius eye, comprising three or four closely applied ocelli. The nauplius eye, characteristic of crustacean larvae, is absent in many adults; and some groups lack the compound eyes. Nitrogenous excretion is via a pair of maxillary glands. Most of the 31 400 species are marine, but there are many freshwater species, and a relatively small number have invaded the land. A few marine species are parasites of other Crustacea; and one group, the Cyamidae, are ectoparasites of whales (whale lice). The first representatives of the group are known from Cambrian rocks.