Hymen

A very delicate, porous membrane of silica covering the opening of an areola or an alveolus. 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. Found in many raphid genera such as Cocconeis and Neidiopsis. Visible only with the SEM. Plural form is hymenes.

See also cribrum, a similar structure with larger pores found in many centric and some raphid diatoms.

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.