From the tips of the gametophore stems or branches develop the sex organs of the mosses. The female organs are known as archegonia ("sing." archegonium) and are protected by a group of modified leaves known as the perichaetum (plural, perichaeta). The archegonia are small flask-shaped clumps of cells with an open neck (venter) down which the male sperm swim. The male organs are known as antheridia ("sing." antheridium) and are enclosed by modified leaves called the perigonium ("pl." perigonia). The surrounding leaves in some mosses form a splash cup, allowing the sperm contained in the cup to be splashed to neighboring stalks by falling water droplets.