Respiratory system of gastropods

In gastropods in many ancient lineages, the gills are bipectinate, having an overall shape that is similar to a bird's feather, with narrow filaments projecting either side of a central stalk.

The water current to supply these gills is evacuated through a slit or notch in the upper surface of the shell, below which the anus opens.

Bipectinate gills have to be supported by membranes, and these can become fouled with debris and sediment, restricting such gastropods to relatively clean-water environments, such as water flowing over solid rock.

This type of gill is firmly anchored to the mantle wall along its length, with a single row of filaments projecting down into the water stream.

In one amphibious group, the Ampullariidae, the mantle cavity is divided into two, with a unipectinate gill on one side, and a lung on the other, so that these snails can respire using air or water.

This type of arrangement is found in the bubble shells and also in the sea hares, which also possess an exhalant siphon which sends fouled water away from the body.

Instead, the upper surface of the body has numerous club-shaped or branched projections called cerata that function as secondary gills.

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.

Some of these possess secondary gills within the mantle cavity, while others must regularly return to the surface to breathe air, keeping their pneumostome closed while they are submerged.