peristome

Peristome is a term used in biology to denote a rim, border, or surrounding structure around a mouth-like opening. The word derives from Greek peri-, around, and stoma, mouth. The exact form and function vary among organisms, but the central idea is a ring or edge encircling an opening.

In bryology, the peristome refers to the ring of tooth-like projections around the opening of the moss capsule (sporangium). After the operculum, or lid, falls away, the peristome teeth often move in response to humidity to regulate spore discharge, helping to control release according to environmental conditions.

In mollusks, particularly gastropods, the peristome is the edge of the shell aperture surrounding the opening through which the animal extends its body. The peristome can be simple or reinforced and may be thickened, reflected, or ornamented, aids in shell structure, and can be a feature used in species identification.

In ciliates and some protozoa, the peristome denotes the region around the cytostome, or mouth opening. This area may form a ring or apron that channels food particles toward the oral opening and is often relevant in taxonomic descriptions.

In a broader sense, peristome describes any surrounding rim around an orifice or mouth-like structure, across diverse groups of organisms. The specific anatomy and function reflect the needs of the organism and the nature of the opening it encircles.