Most freshwater snails have a distinctive head with two conspicuous tentacles and a small black eye at the base of each, or eyes on the tips of the tentacles. The land snails and slugs have four tentacles with the eyes on the ends of the larger pair. When feeding or moving about, a snail can come almost entirely out of the shell, which is permanently attached to and carried on its back. On the underside of the head is the mouth which, when feeding on the glass side of an aquarium, can be seen slowly opening and closing. Inside is a horny file-like tongue with which it scrapes the algae and other growths from the surfaces of rocks, submerged plants, and muddy or sandy bottoms.
Some freshwater snails have gills by which they obtain oxygen from the water.
Others, such as most of our common pond snails, and the land snails and slugs, breathe with a lung and those living in water must come to the surface frequently. Land snails, slugs and air-breathing water snails do not have separate sexes. Instead, each animal has both male and female organs. The land snails and slugs lay their eggs, about 50 at a time, in summer or early autumn, among grass roots or other damp places. The air-breathing water snails lay tiny eggs, imbedded in clear blobs of jelly, at frequent intervals throughout the year.