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Cephalocarida (phylum Arthropoda, subphylum Crustacea) The most primitive class of crustaceans, first described in 1955, cephalocarids are shrimp-like and small (all less than 4 mm long). The body comprises a semicircular head and a long trunk of 19 segments, of which only the anterior nine bear limbs. Eyes are absent and both pairs of antennae are short. All the trunk limbs are similar, and they resemble the second pair of maxillae. They are unusual in that each bears a pseudopipodite. Cephalocarids are hermaphroditic, and live in bottom silts which they filter for organic matter. The class comprises four genera, with seven species, and has been found in coastal waters off eastern and western N. America, the W. Indies, and New Zealand.