Didinium

Didinium is a genus of ciliate. They are free-living carnivores, mostly found in fresh and brackish water environments, although at least three marine species are known. Their diet consists primarily of Paramecium, but some species such as D. gargantua feed on other protists including dinoflagellates, cryptomonads, and green algae.

Structure
Didinium have two ciliary bands, called pectinellae, encircling the organism. This distinguishes them from the related genus Monodinium, which only have a single band except during cell division. The pectinellae are used to move through water by rotating the cell around its axis. Didinium are rounded, oval, or barrel-shaped and range in length from 40 to 200 µm. At the anterior end, a cone-shaped structure protrudes which encloses the cytostome or "mouth" opening, as found in other haptorian ciliates. The dimension of cone varies among the different species. The nucleus may also appear curved in an arched, U-shape. Didinium reproduce via binary fission asexually, or sexually via conjugation.

D. Nasutum
Much of what is described for this genus is based on the extensive studies of a single species, Didinium nasutum. A voracious predator, D. nasutum, employs specialized structures called toxicysts (some sources refer to these structures as trichites) to ensnare and paralyze the ciliary movement its prey, Paramecium. Once captured, the Paramecium, is engulfed whole by D. nasutum through its anterior cytostome.

In the absence of its Paramecium prey, D. nasutum will encyst, lying dormant within a protective coating. The cysts have been shown to remain viable for at least 10 years. When the cyst form of D. nasutum is exposed to its exclusive Paramecium food source it will excyst. At least one laboratory study showed that other conditions can trigger encystment in D. nasutum, such as the build-up of excessive waste products due to a high-density population.

Phylogenetic analysis
Phylogenetic analysis of the ciliates has presented a challenge to researchers due to the presence of both a "germ line" micronucleus containing the genes passed from generation to generation and a "somatic" macronucleus within each organism. Products of conjugation, micronuclear meiosis and fusion to form the zygote are followed by mitotic division and the formation of a macronucleus. Throughout this complex process the chromosomes undergo many processes that affect their composition, making phylogenetic analysis more difficult. In fact, at least two articles conclude that the Haptoria are not a monophyletic group.