Basically all molluscs breathe by gills that are called ctenidia (comb-gills) because of their comb-like shape. In terrestrial molluscs this respiration organ is reduced, but still respiration takes place in the pallial cavity. That is why it is also called the snail's respiratory cavity.
Externally the mantle cavity is recognised by its large entrance opening, the respiratory opening or pneumostome. It can be seen on the right side of the mantle in the shell aperture, opening and closing periodically. When the snail is not breathing, it closes its pneumostome with a circular muscle, to avoid losing to much water by evaporation.
To inhale, the snail widens the respiratory cavity by lowering the cavity floor. Due to the resulting low pressure, air rich in oxygen flows into the cavity (see illustration on the right).
Gas exchange takes place at the cavity's roof. There the tissue is particularly thin, so oxygen (O2) diffuses into the blood fluid circulating there. In the opposite direction carbon dioxide (CO2) diffuses from the blood into the air, following the gradient of gas contained in air and blood respectively. To assist this gas exchange, the snail closes the pneumostome and raises the cavity floor, thus increasing pressure inside the pallial cavity and the rate of oxygen diffusion into the blood.
To exhale the depleted air, the snail finally opens the pneumostome and raises the cavity floor to press out the air. So the respiratory cavity floor performs the same task as does the human diaphragm in respiration.
This described respiratory organ of the Roman snail, or alike, is called its lung, in analogy to the human lung, though it resembles a single alveole, more than the whole organ of a vertebrate's lung. Because it breathes oxygen from dry air with such a snail lung, is placed systematically in the group of lung snails or pulmonate snails (Pulmonata). Those not only live on dry land, but also in fresh water. There they are optimally adapted to the sometimes changeful environmental conditions.