- Any of a phylum (Apicomplexa) of protozoans that are parasitic in animals, especially animal bloodstreams, and are distinguished by a variety of organelles, including fibrils and microtubules, located at one end (called the apical end) of the cell. These organelles help the apicomplexan invade an animal cell. Apicomplexans form spores and reproduce sexually in an alternation of generations; many have complex life cycles and are transmitted to animals hosts by bloodsucking insects. Apicomplexans include the organisms that were once classified as sporozoans, including the protozoans that cause malaria and toxoplasmosis.
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