VORTICELLA, the bell-animalcule, a Protozoan genus of the large family V orticellidae belonging to the Peritrichous Infu soria (q.v.) characterized by the bell-shaped body, with short oral disc and collar, attached by a hollow stalk, inside and around which passes, attached spirally, a contractile bundle of myonemes. By their contraction the stalk is brought into the form of a corkscrew, and the animal is jerked back near to the base of the stalk. As soon as the contraction of the thread ceases, the elasticity of the stalk extends the animal to its previous position. On fission, one of the two animals swims off by the development of the temporary posterior girdle of membranelles, the disc being retracted and closed over by the collar, so that the cell is ovoid; on its attachment the posterior girdle of cilia disappears and a stalk forms. The other cell remains attached to the old stalk.
In the allied genera Carchesium and Zoothamnium the two pro duced by fission remain united, so that a branching colony is ultimately produced. The genus is a large one. The gametes in conjugation differ from one another, one being attached, the other free. Each pair fuses together completely and permanently.