ORDER VII. SEISONACEAE.—Marine Rotifers parasitic on the Crustacean _Nebalia_; males resembling the females. Body elongated, with a slender retractile neck, a much reduced disc, an elongated foot with a terminal perforated disc as in _Callidina_. Trophi virgate exsertile. Genito-urinary cloaca opening at the base of the neck in the male, at the hinder end of the body in the female. Intestine complete (_Seison_) or blind (_Paraseison_).[275] {226}Fam. 24. Seisonidae: _Seison_ Grube; _Paraseison_ Plate; _Saccobdella_ Van Beneden and Hesse. HABITS.—The habitat of Rotifers is well known to the student of pond life. Every dip from a greenish pool will give us a supply, if there be not an excessive contamination by manure; and such pools give us some of the largest and most beautiful forms, such as _Hydatina_ and _Brachionus_, swimming about among the fibrous Algae and feeding on the organic débris among them. Almost any organic infusions freely exposed to the open air will yield Ploima shortly after the active putrefaction is completed. The finer water-weeds yield most of the beautiful tubicolous forms. A whole group of species and genera are quasi-pelagic in fresh and salt water, constituting a large proportion of the "plankton" or floating life near the surface; and some of these are found in deep water or in the depths of the lakes. Among them are the Asplanchnidae, Triarthridae, and Anuraeidae. A number of Loricates, such as _Notholca_ and _Eretmia_, are armed with long spines, which doubtless render floating easier. Among tubicolous forms _Conochilus volvox_ and _Lacinularia racemovata_ have this pelagic habit, forming floating globular or ovoid colonies, and two species of _Floscularia_ also float freely in their tubes. The following forms occur in salt or brackish water,[276] those marked with an asterisk (*) also occurring in fresh water:—