Canyon Park

During the early stages of development, the sporophyte is completely surrounded by a tough protective covering called the calyptra. As the sporophyte grows and enlarges, the calyptra is carried up into the air and eventually comes to sit atop the capsule like a tiny hood (a). When the sporophyte is mature, the calyptra is shed and the capsule is revealed (b). At the tip of the capsule is a lid (operculum) which, prior to spore dissemination, falls away exposing the so-called peristome teeth, a set of structures, often delicate and thread-like in appearance, that form a ring around the mouth (rim) of the capsule (c). The peristome teeth are perhaps the most characteristic feature of the mosses; usually composed of cell wall remnants, they respond to changes in the humidity of the atmosphere. Under conditions of low humidity, the teeth dry out and splay away from the mouth of the capsule, thus, allowing the commonly more than 50,000 spores within to be gradually released.

From: http://www.bio.umass.edu/biology/conn.river/mosses.html