A relative of the paramecium, Urocentrum is a rotund ciliate slightly bifurcated by a two distinct bands of cilia and sporting a tufted “tail” of fused cilia on its posterior. The organism spins on this tail, swimming rapidly in a slightly irregular spiral. It commonly occurs in decaying pond vegetation in which it feeds on bacteria and particles of organic matter.
Urocentrum belongs to the class Nassoporea in the phylum Ciliophora; the ciliates, of which there are approximately 8,000 species, are generally considered to be the most evolved and complex of the protozoans. The cell surface is covered with hundreds of hairlike structures called cilia that are arranged in rows. The cilia beat in synchronized waves, propelling the organism through the water. Most ciliates possess an oral cavity, or cytostome, through which food enters the cell. In some ciliates, the cilia around the cytostome have become modified into sheets called membranelles, which create a feeding current and act as a sieve to trap food particles. Ciliates possess two types of nuclei, a large nucleus, or macronucleus, and one or more small nuclei, or micronuclei. Reproduction is primarily asexual, by binary fission, but sexual reproduction also occurs by conjugation, a process by which two organisms exchange nuclear material.