Introduction
The subphylum Crustacea was recently expanded to accommodate the Hexapoda (insects and allies), which are now considered a deep branch within the crustacean evolutionary tree. As a result of this reclassification, the Crustacea now comprises over 1,000,000 known species. Most of the more familiar crustacean groups such as the Decapoda, Stomatopoda, Isopoda, Copepoda and Cirripedia are now included in the superclass Multicrustacea within the subphylum.
Diagnosis
Body composed of head and trunk, trunk differentiated into thorax and abdomen or comprising undifferentiated segments; if trunk undifferentiated then occurring in aquatic environments (unlike Myriapoda which also have undifferentiated trunk but are terrestrial); head with 3-5 pairs of free appendages including first antennae, second antennae (absent in Hexapoda), mandibles, first maxillae and second maxillae (fused to form labium in Hexapoda).
