 If two layers are present, the outer layer is called the exoperidium and the inner layer the endoperidium.If three layers are present, they are the exoperidium, the mesoperidium and the endoperidium.In the simplest subterranean forms, the peridium remains closed until the spores are mature, and even then shows no special arrangement for dehiscence or opening, but has to decay before the spores are liberated.