How land snails breathe

Land snails in a garden are pulmonates. They breathe with a lung, a pocket in the mantle lined with blood vessels, not with gills. The opening of that pocket is the pneumostome, a small hole on the right side of the body, just under the shell lip in a right-handed snail. Sit still and you can see it open and close. It is not a wound. The snail controls it to limit water loss as much as to move air.

Oxygen passes into the blood across the roof of the lung. Snail blood uses hemocyanin, a copper-bearing pigment, so it is clear or faintly blue rather than red. The heart lies nearby and can sometimes be seen beating through the thin shell of a small individual, a flicker behind the last whorl. None of this is fragile in the way a diagram makes it seem. It is the ordinary machinery of an animal that left the water and kept a modified mantle cavity as its lung.

Aquatic snails divide. Some, including many freshwater pond snails and bladder snails, are pulmonates too. They rise to the surface and open the pneumostome to refill the lung, which is why you see them hanging under the film of a pond. Others, including most marine snails and some freshwater groups, have gills and take oxygen from the water. A few can manage some of both. The word “snail” describes a shape and a way of moving. It does not describe a single way of breathing.

In dry air the pneumostome stays shut longer. Behind a sealed epiphragm, the temporary cover a snail builds across the aperture in drought or winter, gas exchange slows and metabolism falls with it. The animal is not holding its breath so much as needing very little. Rain, or a thaw, is what opens the system again.