The respiratory system of gastropods varies greatly in form. These variations were once used as a basis for dividing the group into subclasses. The majority of marine gastropods breathe through a single gill, supplied with oxygen by a current of water through the mantle cavity. This current is U-shaped, so that it also flushes waste products away from the anus, which is located above the animal's head, and would otherwise cause a problem with fouling.
The sea slug Pleurobranchaea meckelii respires using a gill which is visible in this view of the right side of the animal
Freshwater snail Melanoides tuberculata. Arrows are pointing to the pallial tentacles for breathing.
The dorid nudibranch Chromodoris joshi has a rosette of gills far back on the body. Nudibranch means "naked gills". Near the front of the animal are the two rhinophores.
In this image of an individual of Cepaea nemoralis, a pulmonate land snail, the pallial lung is visible through the translucent shell as the brightest yellow part of the body whorl
Pulmonata
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Pulmonata or pulmonates is an informal group of snails and slugs characterized by the ability to breathe air, by virtue of having a pallial lung instead of a gill, or gills. The group includes many land and freshwater families, and several marine families.
Shells of pulmonate stylommatophoran snails in a museum collection
An artistic but scientifically incorrect version of various European land snails and slugs (one species here is not a pulmonate), their food plants and fungi, and a beetle that eats mollusks, bottom right.
Examples of Pulmonata: Achatina fulica top right, Bielzia coerulans top left, Praticolella berlandieriana center right, Megalobulimus oblongus in the center, Euglandina rosea center left, Helix pomatia bottom right & Ashmunella levettei bottom left