Thin membrane or skin protecting subdermal layers

noun

1. An outer membrane of some protozoans (such as euglenoids or paramecia)
2. A film that reflects a part of the light falling upon it and transmits the rest and that is used for dividing a beam of light (as in a photographic device)
3. A slimy or gelatinous film that contains bacteria, yeasts, or both and that may form on the surface of a liquid (such as beer, wine, or kombucha) during the process of fermentation