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a genus of parasitic protozoans of the class Flagel-lata. The body, which reaches 1 mm in length, is not completely symmetrical, is leaflike, and covered with numerous flagella. The ectoplasm and endoplasm are well demarcated and sometimes have dozens, even hundreds, of nuclei. There is no mouth opening; feeding is by osmosis. Reproduction is asexual (binary division) and sexual (copulation).
Several dozen species of Opalina have been described. They usually live in the intestines of amphibians; they are found less frequently in the intestines of fish and reptiles. The sexual process occurs in the spring in the gut of a tadpole. Cysts that are evacuated from the intestine of an animal host, such as a frog, are deposited on the bottom of a body of water, where they are swallowed by tadpoles that have just hatched. In the tadpoles’ intestines, small opalinids hatch from the cysts, which divide to form uninucleate sexual individuals, or gametes. The gametes fuse in pairs, and multinucleate opalinids develop from the zygotes.
Until recently, the Opalina were considered to be a subclass of Infusoria. However, opalinids lack the two principal characteristics of infusorians: two types of nuclei and the sexual process of conjugation.