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Mezozoa (mĕz´əzō´ə, mē´zə–), name of an animal subkingdom and also of the subkingdom's only phylum. The mezozoans are simple parasitic marine wormlike animals of only 20 to 30 cells, which are differentiated only into reproductive cells and ciliated cells. They are placed in their own subkingdom because they are so unlike any other animals.