[Crustacea] The phase of crustacean larval development characterised by two pairs of functional appendages (the first and second antennae). The mandibles are reduced and no longer serve as swimming appendages. Buds of the first and second maxillae and maxillipeds are present. A carapace covers the anterior portion of the body, and an unsegmented abdomen bears posterior spines. In species which shed their eggs freely into the water there is one metanauplius stage. In species which brood their eggs there is a newly-hatched form, the pseudometanauplius, and the metanauplius form. These forms may be separated by the moult of a thin cuticle soon after hatching or the pseudometanauplius may develop into the metanauplius without moulting.