The air fern (Sertularia argentea) is a species of marine animal in the family Sertulariidae. It also known as the sea fir and Neptune plant. These so-called "ferns" are dead and dried colonies of hydrozoans, colonies of marine hydroids, class Hydrozoa, phylum Cnidaria. Hydroids are related to corals and jellyfish. The fernlike branches of S. argentea are composed of many small, chitinous chambers where individual animals once lived. When the colony was alive, a polyp with numerous tentacles occupied each of the chambers, called hydrotheca. Sometimes dried bryozoa are sold as "air ferns."