Slug, the common name of various mollusks that are related to snails, but lack a shell or have only a fragmentary shell embedded under the skin. There are both land- and sea-dwelling slugs.

Land slugs are slow-moving animals with soft bodies. They are one to eight inches (2.5 to 20 cm) long, depending on the species. Projecting from the head are two pairs of tentacles; the front pair is sensitive to touch, the other pair bears eyes. Land slugs hide in moist, dark places during the day and come out at night to feed. Some eat insects and earthworms but most feed on leafy plants and often become serious garden pests. In winter, slugs burrow into the ground to hibernate.

Sea slugs (a term that refers to both nudibranchs and related animals called sea hares) are found in all oceans, where they either crawl on the bottom or swim with sinuous movements. There are about 2,500 species. They range in length from 3/8 of an inch to 39 inches (1 cm to 1 m) and are often brightly colored. Most sea slugs have two pairs of tentacles: (1) cephalic tentacles (sense organs) located just behind the mouth and (2) rhinophores (respiratory organs) located farther back on the head. Sea slugs are hermaphroditic (each individual has both male and female reproductive organs).

Most nudibranchs have brilliantly colored, fingerlike projections (called cerata) on their backs; these projections are respiratory organs. Immature nudibranchs have delicate external shells, which they lose as adults. Nudibranchs are carnivores, feeding on such things as sponges, sea anemones, barnacles, and fish eggs.

Sea hares have thin internal shells and rhinophores that are more prominent than those of nudibranchs. Sea hares eat only algae.

Which Snails Don’t Have Shells?

Slugs look like snails without shells, and that’s exactly what they are. In fact, most slugs go through life without shells. This might seem dangerous for animals that move so slowly. But slugs have some ways to stay safe that really work.

Many land and sea slugs have colors that blend in with their surroundings. This makes it hard for predators to see them. Other slugs,have bright colors that warn predators to stay away. Their brilliant colors let enemies know that they don’t taste good.

Sea slugs have other tricks to escape their enemies, too. Some can simply swim away. Others release a foul smell that usually keeps an enemy from getting too close.

Slugs belong to the class Gastropoda. Land slugs are of the order Stylommatophora. Nudibranchs make up the order Nudibranchia; sea hares, the order An asp idea.