Metazoa

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Metazoa

animal

Also known as: metazoan

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Last updated

June 22, 2026

Quick Summary

Metazoa, also known as animals, are multicellular eukaryotic organisms. Unlike plants and fungi, animals have developed muscles, enabling mobility, which has driven the evolution of specialized tissues and organ systems.1

The animal kingdom is incredibly diverse, with over three-quarters of Earth's species represented. This diversity stems from their ability to move, allowing them to pursue food, mates, and escape predators, leading to a wide range of adaptations in feeding, defense, and reproduction.1

Animals can be broadly classified into two subkingdoms: Parazoa (like sponges), which have cells that specialize but lack distinct tissues and organs, and Eumetazoa, which possess true tissues and organs. This fundamental organization, established during embryonic development, limits the size and complexity of animals and includes variations in symmetry, tissue layers, and body cavities.234

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Ediacaran Period
…evolution of multicellular animals (the metazoans). The predominant Ediacara fauna in the fossil record is a group of unusual soft-bodied (invertebrate) forms that predated the Cambrian explosion—the unparalleled emergence of organisms between 541 million and approximately 530 million years ago that included representatives of many
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…evolution of multicellular animals (the metazoans). The predominant Ediacara fauna in the fossil record is a group of unusual soft-bodied (invertebrate) forms that predated the Cambrian explosion—the unparalleled emergence of organisms between 541 million and approximately 530 million years ago that included representatives of many
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reproductive behaviour
Protozoans and sponges
In the lower metazoans (multicellular organisms), reproduction is also by both asexual and sexual means. As befits their sessile life-style and low population densities, sponges that reproduce sexually are usually hermaphroditic; that is, each individual is capable of producing both sperm and eggs, but often at different times…
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In the lower metazoans (multicellular organisms), reproduction is also by both asexual and sexual means. As befits their sessile life-style and low population densities, sponges that reproduce sexually are usually hermaphroditic; that is, each individual is capable of producing both sperm and eggs, but often at different times…
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phylogeny
Animal evolution
…origin of multicellular animals (metazoans) was long dominated by German embryologist Ernst Haeckel’s theory that the original metazoan ancestor was a spherical protozoan that was structurally similar to the cnidarians (e.g., jellyfish and corals). Currently there are two alternative explanations. The first traces metazoans back to
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…origin of multicellular animals (metazoans) was long dominated by German embryologist Ernst Haeckel’s theory that the original metazoan ancestor was a spherical protozoan that was structurally similar to the cnidarians (e.g., jellyfish and corals). Currently there are two alternative explanations. The first traces metazoans back to
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Precambrian
Ediacaran fossils
However, more-primitive metazoans without skeletons—the Ediacara fauna—appeared earlier (more than 600 million years ago), after the end of the Varanger-Marinoan ice age at 580 million years ago and before the onset of the Cambrian Period at 541 million years ago. They are found as impressions of soft-bodied,…
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However, more-primitive metazoans without skeletons—the Ediacara fauna—appeared earlier (more than 600 million years ago), after the end of the Varanger-Marinoan ice age at 580 million years ago and before the onset of the Cambrian Period at 541 million years ago. They are found as impressions of soft-bodied,…
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