Physiology of Finfish and Shellfish (2+1)

2.1.37. Pulmonates

The pulmonates have lost their gills and adapted the mantle cavity into a lung. The lung has a single opening on the right side, called the pneumostome, which either remains permanently open, or opens and closes as the animal breathes. The roof of the lung is highly vascularised, which save sa serface to tailited the gas exchange. 

Despite the presence of a lung, many pulmonates spend much of their life is underwater condition. Some of these possess secondary gills within the mantle cavity. While other must regularly return to the surface to breathe air, keeping their pneumostome closed while they submerge. A few freshwater species living in deep lakes have lost the ability to breathe air and flood their mantle cavity with water.

The majority of pulmonates, however, are fully terrestrial most have the typical lung arrangement described above, but in the Athoracophoridae, the mantle cavity is replaced by a series of blind tubules, while the Veronicellidae respire through their skin, and have lost the lung altogether.

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. Those 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.