Gastropoda is a very diverse group of mollusks, without many common features among the 80.000 different species. They live in a wide range of habitats, many are terrestrial, others live in freshwater, in the tidal zone or in abyssal depths.
They have a single foot to keep in contact with the ground. Some has the heart positioned in front of the gills, others behind. This was traditionally a feature used to define the taxonomy of gastropods. Almost every marine species utilize gills for respiration, while terrestrial and freshwater species use lungs.
In common language we use the word snail about gastropods with a shell large enough to allow the snail to fully contract into it. Those without are called slugs. Most marine snails and slugs in the Norwegian fauna are members of one of the following subclasses: