Definition of "peristome"

peristome

noun
plural peristomes
1. (botany) One or two rings of tooth-like appendages surrounding the opening of the capsule of many mosses.
  Quotations
  Its removal does not, however, leave the mouth of the capsule wide open, for around the margin are two circles of pointed teeth forming the peristome.
  1911, “Bryophyta”, in Encyclopædia Britannica Eleventh Edition
2. (zoology) The parts of or surrounding the mouths of numerous invertebrates.
  Quotations
  We can distinguish therefore in the body of a polyp the column, circular or oval in section, forming the trunk, resting on a base or foot and surmounted by the crown of tentacles, which enclose an area termed the peristome, in the centre of which again is the mouth. […] The mouth may be level with the surface of the peristome, or may be projecting and trumpet-shaped.
  1911, “Polyp”, in Encyclopædia Britannica Eleventh Edition
3. The lip, or edge of the aperture, of a spiral shell.