Eumalacostraca is a subclass of crustaceans, containing almost all living malacostracans, or about 40,000 described species. The remaining subclasses of crustaceans that are not in Eumalacostraca are the Phyllocarida and possibly the Hoplocarida. Eumalacostracans have 19 segments (5 cephalic, 8 thoracic and 6 abdominal). This arrangement is known as the "caridoid facies", a term coined by William Thomas Calman in 1909. The thoracic limbs are jointed and used for swimming or walking. The common ancestor is thought to have had a carapace, and most living species possess one, but it has been lost in some subgroups.

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