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The pneumostome is a feature (the respiratory opening) of the external body anatomy of an air-breathing land slug or land snail.
It is an opening in the right side of the mantle of a Pulmonata ordered or subclassed slug (suborder Stylommatophora) or snail. Air enters through the pneumostome into the animal's single lung, the air-filled mantle cavity.
The pneumostome is often much easier to see in slugs than in snails, because of the absence of a shell. The pneumostome is usually very clearly visible on the right side of the animal when the pneumostome is wide open. However, the position of the pneumostome is often not at all easy to discern when the pneumostome is closed.
The pneumostome opens and closes in a cyclical manner. The frequency of pneumostome closing and opening is typically less than 0.5 closures per minute in fully hydrated slugs and snails. The rate of closures per minute increases the more dehydrated the slug is.
The images in the gallery below show the position of the pneumostome in three different families of slugs:
 Arionidae, (Arion ater), the pneumostome is just anterior to the mid-point of the mantle |
 Limacidae, (Bielzia coerulans), the pneumostome is just posterior to the mid-point of the mantle |
 Milacidae, (Tandonia budapestensis), the pneumostome is just posterior to the mid-point of the mantle |
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