Although not true pulmonates, some operculate land snails, such as the Cyclophoridae have also lost their gills, and developed a vascularised lung from the mantle cavity. These groups lack a pneumostome, having a much larger opening to the cavity. Since this opening is usually closed by the operculum when the animal retreats into its shell, there is often a separate breathing tube or a notch in the shell to allow air in and out.