peristome A single or double ring of setae (teeth) which fringes the mouth in many types of moss capsule; the peristome can be seen when the lid of a ripe capsule falls or is removed. The setae are hygroscopic (absorb moisture from the air). The fibres from which they are made are coiled helically; changes in humidity cause changes in the lengths of the fibres, twisting the capsule and producing jerky movements that aid in the dispersal of the spores.

peristome
1. A ring of toothlike structures around the opening of a moss capsule. The teeth tend to bend and twist in dry weather, so opening the mouth of the capsule and allowing the spores to escape. In wet weather they close over the opening of the capsule.

2. The area around the mouth in many invertebrates and some protoctists. It sometimes assists in food collecting. Examples are the spirally ciliated groove around the mouth of some ciliate protozoans and the first segment of the earthworm.

peristome (adj. peristomal) In Echinoidea, the area around the mouth, covered in a leathery skin studded with small plates.

peristome In invertebrates, the mouth cavity.