Pneumostomes

Pneumostomes are external pores or openings found in certain terrestrial invertebrates, most notably in land snails and slugs. These openings serve as a vital respiratory structure, allowing for the intake of air and the expulsion of carbon dioxide. In gastropods, the pneumostome is typically a single, slit-like aperture located on the side of the mantle, near the shell opening. When the animal is active and breathing, the pneumostome can be observed to open and close.

The internal structure connected to the pneumostome is a lung cavity, which is essentially a highly vascularized space within the mantle. Oxygen diffuses from the inhaled air across the thin membrane of the lung cavity into the snail's or slug's hemolymph, while carbon dioxide is released from the hemolymph into the lung cavity to be expelled through the pneumostome. The size and visibility of the pneumostome can vary between different species of gastropods. In some, it is quite prominent, while in others, it may be less conspicuous. The presence of a pneumostome is a key characteristic distinguishing terrestrial pulmonate gastropods from aquatic gastropods, which typically respire using gills.