Slug
Slug is a general term for a gastropod mollusc which has no shell, or just a small internal shell.
Slugs belong to several different families which also include snails with shells. Snails are gastropods with a coiled shell large enough for the animal to pull back inside. The families of land slugs are not very closely related, despite looking similar. The shell-less slug type is an example of convergent evolution, and so the category "slug" is not a taxonomic category.
The name "slug" is used for air-breathing land slugs, while the marine forms are usually known as sea slugs. Land gastropods with a shell that is too small to hide the body are sometimes called semislugs.
Body parts and behaviour
Like land snails, most slugs have four 'feelers' or tentacles on their head. The upper two (the "optical tentacles") see light and have eyespots at the ends, while the lower two (the "sensory tentacles") are used for smelling. The tentacles are retractable and can regrow if lost.
On top of the slug, behind the head, is a body part called the mantle, and under this are the genital opening and anus. On one side (almost always the right side) of the mantle is a breathing hole, which is easy to see when open, but difficult to see when closed. Inside the mantle in some species is a very small, flat shell.
The bottom of a snail is called a "foot". Like other snails, a slug moves by rhythmic waves of muscle contraction on the bottom of its foot. At the same time, it secretes a layer of mucus on which it travels, which helps prevent damage to the foot. Around the edge of the foot is the 'foot fringe'.
Some slug species hibernate underground during the winter in places with cold winters, but in other species, the adults die in the autumn.
Slug Media
An active Ambigolimax slug in Fremont, California
Deroceras juvenile with eggs of another (larger) slug species, most likely Arionidae
Large slug near Manali, India
Lehmannia sp. feeding on a small fruit in Mexico City
A slug retracts its head and contracts its body after being touched
The alarm response posture of the Kerry slug, which is found only in this species
