Vorticella

Vorticella, the Bell-Animalcule, a genus of Peritrichous Infusoria characterized by the bell-shaped body, with short oral disk 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, the thread being now on the shorter, i.e. the inner, side of the turns; 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 disk 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 produced by fission remain united, so that a branching colony is ultimately produced. The genus is a large one, and many species are epizoic on various water animals.