As familiar gardeners’ pest, slugs and snails belong to a class of animals called Gastropoda, which also include the sea snails and slugs, as well as freshwater snails, bivalves and limpets. In turn this group fall under the classification phylum Mollusca, which are the molluscs and include octopus and squid, amongst many other species. The word Gastropod comes from the Greek language translating to “gastro” meaning stomach and “pod” meaning foot, referring to the animal’s foot being positioned below its guts.
There are around 46 species of terrestrial slug in Britain and Ireland, and about 120 species of land snail. The difference between slug and snail is initially obvious, as snails have a coiled shell made from calcium carbonate which they can retreat inside of when threatened or to avoid dessication in hot weather, where as slugs lack an outer shell. Slugs evolved from snails, reducing their shell to a plate which is found internally under the skin towards the tail end. Both slugs and snails are air-breathing, and have a lung within the mantle cavity, and in slugs can be seen a large breathing pore called the pneumostome which leads to the lung. They also have a large fleshy foot which helps them traverse their environment, and they produce mucus which helps them slide along, and can help with protection when attacked.
Slugs and Snails have voracious appetites, and this is why they are hated by gardeners and growers. They don’t solely feed on the leaves of live plants, but will also eat fruit and vegetables, some prefer decaying plant material or fungi, algae or lichen. Most species of slug will scavenge and feed on other dead or dying slugs and snails, whilst some will attack and kill worms with powerful blade-like teeth.
Many thanks to Ben Rowson and Chris du Feu from iRecord who helped confirm and correct species where appropriate.
Useful AIDGAP reference books:
Slugs of Britain and Ireland by Ben Rowson, James Turner, Roy Anderson and Bill Symondson.
Land Snails in the British Isles by Robert Cameron.
Freshwater Bivalves of Britain and Ireland by Ian Killeen, David Aldridge and Graham Oliver.