Didinium
Didinium is a genus of unicellular ciliates with at least ten accepted species. All are free-living carnivores. Most are found in fresh and brackish water, but three marine species are known. Their diet consists largely of "Paramecium", although they will also attack and consume other ciliates. Some species, such as "D. gargantua", also feed on non-ciliate protists, including dinoflagellates, cryptomonads, and green algae. Appearance and reproduction. "Didinia" are rounded, oval or barrel-shaped and range in length from 50 to 150 micrometres. The cell body is encircled by two ciliary bands, or "pectinelles", an upper band and  a lower band just below the midline. This distinguishes them from the related genus "Monodinium", which have only a single band, except during cell division. The pectinelles are used to move "Didinium" through water by rotating the cell around its axis. At the anterior end, a cone-shaped structure protrudes, supported by a palisade of stiff microtubular rods (nematodesmata). This cone encloses the cytostome or "mouth" opening, as in other haptorian ciliates. The dimensions of this protuberance vary among the different species. The macronucleus is long, and may be curved, horseshoe-shaped or twisted into a shape resembling a figure  eight. A contractile vacuole and anal aperture are in the posterior of the cell. Like all ciliates, "Didinia" reproduce asexually via binary fission, or sexually through conjugation. Section::::"Didinium nasutum". Much of what has been published about this genus is based on numerous studies of a single species, "Didinium nasutum". A voracious predator, "D. nasutum" uses specialized structures called toxicysts to ensnare and paralyze its ciliate prey. Once captured, the prey is engulfed through "Didinium"'s expandible cytostome. While "D. nasutum" is sometimes described as feeding exclusively upon "Paramecium", it has been shown that the organism will readily devour other ciliate  species, including "Colpoda", "Colpidium campylum", "Tetrahymena pyriformis", "Coleps hirtus" and "Lacrymaria olor". Moreover, strains of "Didinium" raised on a "Colpidium campylum" will actually show a preference for a diet made up of that species, as well as a diminished ability to kill and ingest "Paramecia". In the absence of food, "D. nasutum" will encyst, lying dormant within a protective coating. In the laboratory, other environmental stimuli, such as the age of the growth medium or the accumulation of certain metabolic waste products, can also trigger encystment. When the encysted form of "D. nasutum" is exposed to a vigorous culture of  Paramecium, it will excyst, reverting to its active, swimming form. Didinium cysts have been shown to remain viable for at least 10 years. History and classification. "Didinium" was discovered by the eighteenth-century naturalist O.F. Müller and described in his "Animalcula Infusoria" under the name "Vorticella nasuta". In 1859, Samuel Friedrich Stein moved the species to the newly created genus "Didinium", which he placed within the order Peritricha, alongside other ciliates which have a ciliary fringe at the anterior of the cell such as Vorticella and Cothurnia. Later in the century, under the taxonomical scheme created by Otto Bütschli, "Didinium  was removed from among the Peritrichs, and placed in the order Holotricha". In 1974, John. O. Corliss created the order Haptorida, within the subclass Haptoria, for "rapacious carnivorous forms" such as "Didinium", "Dileptus" and "Spathidium". This group has since been placed in the class Litostomatea Small & Lynn, 1981. Genetic analysis of Haptorian ciliates has shown that they do not form a monophyletic group. List of Species. "Didinium alveolatum" Kahl, 1930 "Didinium armatum" Penard, 1922 "Didinium balbianii Fabre-Domergue, 1888 " "Didinium bosphoricum Hovasse, 1932 "Didinium chlorelligerum" Kahl, 1935 "Didinium faurei" Kahl, 1930 "Didinium gargantua" Meunier, 1910 "Didinium impressum" Kahl, 1926 "Didinium  minimum" "Didinium nasutum" (Müller, 1773) Stein, 1859