Slug
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 Red Slug
(Arion
rufus) - red colour
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Slugs are gastropods without shells or with very small shells (which are often internal), in contrast with snails from which they share a common ancestor, and have a prominent shell. Although they undergo torsion (180 degree twisting of internal organs) during development, their bodies are streamlined and worm-like, and so show little external evidence of it. This same basic design developed independently in several different groups, the largest being the sea slugs or nudibranchs. Other slugs are found on land, but their soft, slimy bodies are prone to desiccation, so they are confined to moist environments. Among the various species are the grey field slug, Deroceras reticulatus; the garden slug, Arion hortensis; the leopard slug, Limax maximus; and the banana slug, Ariolimax columbianus.
Like snails, slugs have two pairs of 'feelers' or tentacles on their head. The upper pair--optical tentacles--are light sensors; the lower pair provides the sense of smell. Both pairs are retractable and can be regrown if lost. On top of the slug, behind the head, is the saddle-shaped mantle, and under this are the genital opening and anus. The mantle also has a hole, the pneumostome, for respiration. The slug moves by rythmic muscular action of its foot.
Most slugs eat leaves, fungus, and decaying vegetable material, but some are predators and most also eat carrion including dead of their own kind. Slugs eat using a radula, a rough, tongue-like organ with many tiny tooth-like denticles.
Mucus
Slugs produce two types of mucus: one which is thin and watery, and another which is thick and sticky. Both are hygroscopic. The thin mucus is spread out from the centre of the foot to the edges. The thick mucus spreads out from front to back.
Mucus is very important to slugs as it helps them move around, and contains fibres which prevent the slug from sliding down vertical surfaces. Mucus also provides protection against predators and helps retain moisture. Some species use slime cords to lower themselves on to the ground, or suspend from them during copulation.
Reproduction and life cycle
Slugs are hermaphroditic: having both female and male reproductive organs. Once a slug has located a mate they encircle each other and sperm is exchanged through their protruding genitalia. A few days later hundreds of eggs are laid in holes in the ground. Although some species hibernate over the winter in temperate climates, in most species the adults die in the autumn.
A commonly seen practice among many slugs is apophallation, when one or both of the slugs chew off the other's penis. The penis of these species is curled like a cork-screw and often becomes entangled in their mate's genitalia in the process of exchanging sperm. Apophallation allows the slugs to separate themselves. It is possible that the damaged genitalia regenerate, much like the "optical tentacles."
Various species of slug can also reproduce via tiny "darts" of sperm which they fling in the direction of their mate's genitalia.
Slug genitalia are also some of the most prodigious in the world. Ariolimax dolichophallus, a species of banana slug (dolichophallus meaning "long penis" in Latin) has the largest penis-to-body length ratio of any animal. The record-holding specimen had a body length of 6 inches, with a phallus length of 32.5 inches, well over five times the body length.
Predation, defense and pest control
Frogs, toads, snakes, hedgehogs, and some birds and beetles are natural slug predators. Slugs, when attacked, can contract their body, making themselves harder and more compact and thus more difficult for many animals to get a hold. The unpleasant taste of the mucus is also a deterrent.
Some slugs are notable garden pests and there are various methods of controlling them (see Pest control of slugs), such as slug pellets, beer traps, salt, physical barriers and biological pest controls.
Care should be taken when consuming slugs. In certain cases humans have contracted parasite-induced meningitis from eating raw slugs [1].