VORTICELLA

genus of the ciliate protozoan order Peritrichida, a bell-shaped or cylindrical organism with a conspicuous ring of cilia (hairlike processes) on the oral end and a contractile unbranched stalk on the aboral end; cilia usually are not found between the oral and aboral ends. Vorticellas eat bacteria and small protozoans and live in fresh or salt water attached to aquatic plants, surface scum, submerged objects, or aquatic animals. Although vorticellas are often found in clusters, each stalk is fastened independently. The stalk consists of an external sheath that contains a fluid and a spirally arranged contractile thread. When the vorticella is contracted the stalk thread is shortened, and the sheath is coiled like a corkscrew. Vorticellas reproduce by longitudinal fission. One of the two daughter cells retains the original stalk; the other grows a temporary wreath of cilia at the aboral end and migrates. Propelled by these cilia, the migrant eventually grows a stalk, attaches to a substrate, and loses its temporary cilia. In conjugation one small special migrant (microconjugant) finds an attached vorticella (macroconjugant) and the two conjugants amalgamate completely, forming one organism in a sexlike union that eventually leads to fission.