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. Most of the gill-breathers have separate sexes and bear their young alive.
Some snails have a solid plug to close the shell opening after the animal retreats inside.
The land snails, slugs, and most pond snails have no plug but seal themselves in with a slimy covering when they hibernate to pass the winter, or in order to survive a hot summer drought when ponds go dry. Argonne National Laboratory, Division of Educational Programs, Harold Myron, Ph.D., Division Director.