Slugs belong to several different lineages which also include snails that have shells. The word snail is given to gastropods that have a coiled shell large enough for the animal can retract inside it. The various families of land slugs are not very closely related, despite a superficial similarity in the overall body form. The shell-less condition has arisen many times by convergent evolution, and so the category "slug" is polyphyletic. It is not a taxonomic category.
The common name "slug" is usually given to air-breathing land slugs, while the marine forms are usually known as sea slugs. Land gastropods with a shell that is too small to retract into, are known as semislugs.
Morphology and behaviour[change]
Like pulmonate land snails, most slugs have two pairs of 'feelers' or tentacles on their head. The upper pair is light sensing and has eyespots at the ends, while the lower pair has 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. On one side (almost always the right hand 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, rather flat shell.
Like other snails, a slug moves by rhythmic waves of muscular contraction on the underside of its foot. It simultaneously secretes a layer of mucus on which it travels, which helps prevent damage to the foot tissues. Around the edge of the foot is the 'foot fringe'.
Some slug species hibernate underground during the winter in temperate climates, but in other species, the adults die in the autumn.