Seisonidae is a family of rotifers, found on the gills of Nebalia, a marine crustacean. Peculiar among rotifers, they are gonochoric; males and females are both present and are equal in size. Both sexes are similar with paired gonads. It is considered to have diverged from the other rotifers early on, and in one treatment is placed in a separate class Seisonoidea. They have a large and elongate body with reduced corona. Their muscular system is similar to that of other rotifers: they have longitudinal muscles as well as open annular muscles. Being attached for most of their life, they are semi-sessile, but are capable of detaching and crawl short distances if required.