Land Slugs - Introduction
Upper right: The Leopard Slug, Limax maximus [Fam: Limacidae]. A large northern European slug which has followed humans to most parts of the world. Lower left: Another northern European slug, Testacella haliotoidea, [Fam: Testacellidae] which has also colonised much of the world. Lower right: Internal shells of two limacid slugs [upper] and external shell of Testacella [lower]. Photos: Bill Rudman
Land Slugs have evolved from Land Snails not Sea Slugs. The 'slug shape' is better thought of as a particular type of body design rather than a group of closely related animals. Living on land - out of water - is difficult for soft bodied animals such as molluscs, so for land snails, their shell has been an extremely valuable asset, providing a shelter in which they can hide from attack, and help them from drying out on hot and windy days. For reasons we still don't really understand many different snail families have evolved slug forms, in which the shell is gradually lost. Testacella is an example of a slug with an external shell, which is much reduced in size, is no longer calcified, and can have little practical use. In Limax, on the other hand, the shell is a reduced flattened calcareous internal plate.
The hole in the side of the body of Limax is the pneumostome, a breathing hole which opens and closes as air is drawn into the lung. The 'lung' is a modification of the mantle cavity found in snails. Slugs are usually disliked because of their slimy feel, cause by the copious mucus they secrete from skin glands. This slime is vitally important for slugs as it is valuable in providing then with a smooth path to crawl along, and to prevent them from drying out.
Rudman, W.B., 2006 (June 14) Land Slugs - Introduction. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/landslugs01