UG or land is a general term for any apparently crustless terrestrial gastropod mollusk. The word gas is also often used as part of the common name for Gastropoda mollusks, which have no shells, very small shells or only a very small inner shell, especially sea cucumber and semi SL (this is different from snail, which is applicable to gastropods, whose gastropods have shells large enough to make the soft part of the animal completely retract into the shell).

Land's various taxonomic families form part of several completely different evolutionary lineages, including snails. Therefore, although there are surface similarities in the overall body shape, the families are not closely related. In the past of evolution, the SHELLLESS state has appeared many times independently, so the "group" is a multi lineage.

The body is mainly composed of water, without large-scale shell, their soft tissues are easy to dry. They must produce protective mucus to survive. Because of the wet ground, many species are most active after rain. In dry conditions, they hide in wet places, such as under bark, fallen logs, rocks, and artificial structures (such as flower troughs) to help keep the body hydrated. Like all other gastropods, they distort during development (180 ° internal organ rotation). Inside, the anatomy clearly shows the effect of this rotation - but outside, the body looks more or less symmetrical, except that the pneumothorax knife is located on one side of the animal, usually the right.

Two types of mucus are produced: one is viscous and watery, and the other is viscous. Both are hygroscopic. Thin mucus diffuses from the center of the foot to the edge, while thick mucus diffuses from the front to the back. It also produces sticky mucus that covers the whole animal's body. The mucus secreted by the foot contains fibers that help prevent SL from slipping off the vertical surface. The "traces of mucus" that are left behind have some auxiliary effects: other traces of mucus encountered can identify the traces of mucus produced by one of the same species, which is very useful for finding partners. Along the muddy tracks are also part of some carnivorous Ni hunting behavior. Body mucus provides some protection against predators, for example, it can make it difficult for the beak to pick up and grasp, and the mucus itself can be offensive. Some species of UG, such as the largest Limax, secrete mucus cords to hang a pair during mating.