gastropods The peristome is the margin of the aperture of a gastropod shell. It is the edge of the lip of the shell. This part is sometimes reflected (turned back) or thickened once the snail reaches adult size, and these qualities of the peristome can be diagnostic features of the shell which may aid in identification of the species. In other invertebrates The peristome is any of the structures or sets of parts that surround the mouth or invertebrates such as echinoderms or earthworms. References External links Dictionary definition Plant anatomy Mollusc anatomy Fungal morphology and anatomy