As a class the Gasteropoda have a world-wide range, some forms being exclusively marine, others inhabiting fresh waters, while others, again, live upon the land. Amongst the Prosobranchiates, the entire order of the Siphonostomata, and the majority of the Holostomata, are marine; but, amongst the latter, the Mela-niadae and Paludinidae are confined to fresh waters, and the Cerithiadae and Neritidae include a number of fresh or brackish water forms. The Opisthobranchiates are exclusively marine, mostly littoral in their habits, but occasionally oceanic. The Heteropoda are exclusively marine and pelagic. Lastly, amongst the Pulmonates many forms (such as the Snails and Slugs) are strictly terrestrial, whilst others (Limnaea, Planorbis, Ancylus, etc.) are found in fresh or brackish waters.