Cephalocarids, when first discovered in Long Island Sound, were thought to be the most primitive of living crustaceans. This view, based on much detailed work on Hutchinsoniella macracantha, stemmed from the fact that cephalocarids possessed limbs that were phyllopodous (that is, flattened and lobe like, with the shape maintained by fluid pressure) and were similar in form from the back of the head to the end of the thorax, and were added sequentially during development. More recently cephalocarids have been recognized as an early group within the Crustacea, but most likely arose after the early line leading to remipedes. The subclass Cephalocarida comprises one extant order, Brachypoda; and two families, Hutchinsoniellidae and Lightiellidae. A total of nine species in four genera are currently known.
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