An extremely diverse, species-rich group of arthropods that have inhabited marine environments since the beginning of the Cambrian Period. In some ways, they may be thought of as the marine equivalents of the insects. Within the marine realm the crustaceans occupy as varied a spectrum of habitats as the insects inhabit on land. The Crustacea currently are regarded as a subphylum in the monophyletic phylum Arthropoda, although debate about this rank continues among carcinologists.