The word peristome's derived from the Greek peri, meaning 'around' or 'about', and stoma, 'mouth'. It's a term used to describe various structures in plants and invertebrate animals, such as molluscs, that surround an opening to an organ. Moss peristomes In mosses, the peristome's a specialized structure in the sporangium that allows for gradual spore discharge, instead of releasing them all at once. Most mosses produce a capsule with a lid (the operculum) which falls off when the spores inside… (More on Peristome)