How do land mollusks breathe?
In the air-breathing pulmonates the interior of the mantle cavity is converted into a lung and a pumping mechanism for renewal of air is established. The restriction of the respiratory aperture is necessary. The mantle forms the roof of the cavity and is covered with ridges in which pulmonary veins converging towards the auricle. The floor of the cavity is arched and has a layer of muscles, which contract rhythmically. When the muscles contract, the arch flattens and air is drawn in and at the limit of contraction a valve slides across the pneumostome. When the muscles relax, the cavity decreases in size and exchange of gases with the blood in the roof vessels is facilitated by the increase of pressure of the contained air. Then the pneumostome opens and air is expelled; the subsequent contraction of the floor muscles brings in a fresh supply. This ‘breathing’ is not so regular or so frequent as in a vertebrate; moreover, it may cease altogether in the winter when the snail hibernates.
