The Crustacea are distributed over the whole globe, some forms being terrestrial in their habits, but the majority inhabiting the sea or fresh water. As a rule, the development of the Crustacean fauna is in proportion to the temperature, the higher and larger forms being most abundant in warm regions. The groups of the Cirripedia, Rhizocephala, Xiphosura, and Laemodipoda, are only found in salt water. On the other hand, the Ichthyophthira, Ostracoda, Copepoda, Phyllopoda, Eurypterida (?), Amphipoda, Stomapoda, Isopoda, and Decapoda, are found both in fresh and in salt water. Of these, however, the Phyllopods are principally fresh-water forms, and the Stomapods and Decapods are essentially inhabitants of the sea; whilst the Eurypterids are certainly mainly a salt-water group, though some forms may perhaps have lived in fresh water as well. The Isopoda and Decapoda also include terrestrial forms.