A hymen is a very delicate, porous membrane of silica occluding an areola or an alveolus. It is visible only with SEM. The pores of the hymen may be round or elongated and range from 5-10 nm (0.005-0.010 µm) in their shortest diameter. The plural is hymenes. Hymenes are found in many raphid genera such as Cocconeis and Neidiopsis.

In some raphid diatoms (e.g., Navicula) a hymen forms the only occlusion of an areola, covering the inner aperture. In others such as Diploneis a hymen and a cribrum combine to cover an areola. In this genus the hymen lies beneath the outer cribrum.

Occlusion Summary

hymen - a type of occlusion with very small pores of 15 nm or less in diameter.

velum - a category of occlusion with pores 30 nm or more in diameter.

cribrum - velum that is reticulate or that has regularly arranged pores.

rota - velum of one or more bars; may have a widened area in the center.

vola - velum that is an outgrowth of silica from the wall of an areola.

See each type of occlusion for more detail.

Alternative Terms


Used in some older literature (e.g., Ross et al., 1979).

Hymenes  Bahls2014
Image Credit: Loren Bahls
An SEM image of an internal view of a valve of Neidiopsis hamiltonii, showing hymenes covering the inside of two adjacent alveoli lying horizontally. The hymenes are the regions with closely spaced pores. Scale bar = 0.5 µm.