Slug
Slug is a common name that is normally applied to any gastropod
mollusc that lacks a shell, has a very reduced shell, or has a small internal shell. This is in contrast to the common name snail, which is applied to gastropods that have coiled shells that are big enough to retract into.
Slugs belong to several different lineages that also include snails with shells. The shell-less condition has arisen many times independently during the evolutionary past, and thus the category "slug" is emphatically a polyphyletic one. The various groups of land slugs are not closely related, despite a superficial similarity in the overall body form.
As well as land slugs, there are also many marine slugs and even one fresh
water slug species, but the common name "slug" is most frequently applied to air-breathing land slugs, while the marine forms are usually known as sea slugs. Land gastropods with a shell that is not quite vestigial, but is too small to retract into (like many in the family Urocyclidae), are known as semislugs.
Slugs, like all other gastropods, undergo torsion (a 180º twisting of the internal organs) during development. Internally, slug anatomy clearly shows the effects of this rotation, but externally the bodies of slugs appear rather symmetrical, except for the positioning of the pneumostome, which is on one side of the animal, normally the right hand side.
The soft, slimy bodies of slugs are prone to desiccation, so land-living slugs are confined to moist environments and must retreat to damp hiding places when the weather is dry.
bodies are made up mostly of
water, and without a full-sized shell, their soft tissues are prone to desiccation. They must generate protective mucus to survive. Many species are most active just after rain because of the moist ground. In drier conditions, they hide in damp places such as under tree bark, fallen logs, rocks, and man-made structures, such as planters, to help retain body moisture.
Slugs produce two types of mucus: one which is thin and watery, and another which is thick and sticky. Both kinds of mucus are hygroscopic. The thin mucus spreads from the foot's centre to its edges, whereas the thick mucus spreads from front to back. Slugs also produce thick mucus which coats the whole body of the animal.
The mucus secreted by the foot contains fibres which help prevent the slug from slipping down vertical surfaces. The "slime trail" that a slug leaves behind has some secondary effects: other slugs coming across a slime trail can recognize the slime trail as produced by one of the same species, which is useful in finding a mate. Following a slime trail is also part of the hunting behavior of some carnivorous slugs.
Body mucus provides some protection against predators, as it can make the slug hard to pick up and hold by a bird's beak, for example, and the mucus can be distasteful.
Some species of slug secrete slime cords to lower themselves onto the ground, or to suspend a pair of slugs during copulation.