Slug is a common name for a group of terrestrial snails like molluscs with little or no external shell. Examples of common slugs are Limax maximus, the large garden slug, and Limax agrestis, which eats grain seedlings and is regarded as a farm pest in Europe. Other urban species are Arion circumscriptus and Limax flavus.
Slugs are classified in the gastropod subclass Pulmonata. The pulmonates are those animals of land and fresh water that lack the gills of most snails, but generally have a "lung" formed from a portion of the mantle. Slugs use the whole body integument for exchange of respiratory gases. They tend to occupy places that minimize water loss and temperature extremes, often hidden in the daytime and active at night. Evidence of their nocturnal activity are the slime trails often found on sidewalks in the morning.
Sea slugs are also shell-less snails, but they are much more colorful and varied, and they are classified as class Opisthobranchia, order Nudibranchia. The nudibranchs have a snail-like body, with tentacles on the head, an elongated foot, and pointed tail end. The dorsal surface has projections called cerata, or papillae or branchial plumes, which may look showy or bizarre to us, but not at all unusual to other nudibranchs. These presumably function in the place of gills to increase respiratory surface, but also sometimes serve as camouflage. Most nudibranchs are 1 in (2.5 cm) or less in length, but some Pacific coast species are larger.