A class of the phylum Rotifera which comprises a group of little-known marine animals. They form a single family with about seven species and are found only in Europe. The Seisonacea are epizoic or possibly ectoparasitic on crustacea, especially on members of the genus Nebalia. They have a very elongated jointed body with a small head; a long, slender neck region; a thicker, fusiform trunk; and an elongated foot, terminating in a perforated disk (see illus.). The Seisonacea differ from other rotifers in a number of characteristics. They have a very reduced corona, a quite aberrant type of mastax (fulcrate), paired ovaries but no vitellaria in females, and paired testes in the males. Furthermore, the sexes are of similar size and development; only one type of egg is produced, and it requires fertilization. The Seisonacea are also larger than other rotifers, attaining up to 0.12 in.(3 mm) in length.