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Metazoa usually refers to the animals with fully differentiated tissues, including nerves and muscles. This includes all the animal phyla except the sponges, Trichoplax, and certain parasites. In older texts, Metazoa is often taken to be synonymous with our Animalia, excluding only the protozoa. It may be formalized as a subkingdom, but its treatment varies considerably. Metazoa are distinguished from other Animalia in that they can perform apoptosis. This is a biological recognition that cell differentation is essential to these organisms.
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