|apicomplexan (āp'ĭ-kəm-plěk'sən) Pronunciation Key
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.
any protozoan of the (typically) spore-producing phylum Apicomplexa, which is called by some authorities Sporozoa. All apicomplexans are parasitic and lack contractile vacuoles and locomotor processes. Apicomplexans live within the body cavities or the cells of almost every kind of animal, including other apicomplexans. Some genera are pathogenic: Plasmodium causes malaria, and Eimeria and Isospora cause coccidiosis. Apicomplexans feed by absorbing either dissolved food ingested by the host (saprozoic nutrition) or the host's cytoplasm and body fluids. Respiration and excretion occur by simple diffusion through the cell membrane. In the life cycle, sexual and asexual generations may alternate. Sexual reproduction may immediately precede spore formation. Asexual reproduction is by binary or multiple fission (schizogony).
Learn more about apicomplexan with a free trial on Britannica.com.