In the animal kingdom, slugs and snails belong in the phylum Mollusca, a word meaning 'soft animal'.
The number of different species of mollusc varies, according to the source, but is estimated to vary between 30.000 and 120.000 living species. Members of the group are found in nearly all available biotopes on earth.
The phylum Mollusca is subdivided into 8 classes. Except for the Monoplacophora, representatives of all classes are found in the North Sea.
The class Monoplacophora contains only one single genus (Neopilina ), with a number of species known from abyssal zones in the Pacific, Atlantic and Indian Oceans. All other classes will be introduced in separate chapters in the 'Introduction'.
On this CD-ROM, the classification of the Check List of European Marine Mollusca (CLEMAN) from the M.N.H.N in Paris is used.