The Collaborative International Dictionary of English v.0.48:

Peristome \Per"i*stome\ (p[-e]r"[i^]*st[=o]m), n. [Pref. peri- +
   Gr. sto`ma, sto`matos, mouth.]
   1. (Bot.) The fringe of teeth around the orifice of the
      capsule of mosses. It consists of 4, 8, 16, 32, or 64
      teeth, and may be either single or double.
      [1913 Webster]

   2. (Zool.)
      (a) The lip, or edge of the aperture, of a spiral shell.
      (b) The membrane surrounding the mouth of an invertebrate
          animal.
          [1913 Webster]

WordNet (r) 3.0 (2006):

peristome
    n 1: (botany) fringe of toothlike appendages surrounding the
         mouth of a moss capsule
    2: region around the mouth in various invertebrates