metazoan

Metazoa, or metazoans, is a major subdivision of the kingdom Animalia that comprises multicellular, eukaryotic, heterotrophic organisms. All animals are metazoans. The group ranges from simple sponges to complex vertebrates and is distinguished by multicellular organization with differentiated tissues and typically more complex body plans than single-celled organisms.

Sponges (phylum Porifera) are among the simplest metazoans and lack true tissues, whereas most other metazoans have true tissues. The majority of metazoans are eumetazoans, which possess true tissues. Development usually begins with fertilization and embryogenesis that includes cleavage and gastrulation, establishing germ layers (ectoderm, endoderm, and in most lineages, mesoderm). This gives rise to organs and organ systems.

Body architecture varies widely. Metazoans show symmetry ranging from radial to bilateral, and many have a coelom and segmented bodies. Reproduction is typically sexual, with a larval stage in many groups, but asexual modes occur in some lineages. Metazoans are found in virtually every habitat, from deep oceans to terrestrial ecosystems.

Evolutionary significance: Metazoa is a monophyletic group within the opisthokont lineage, representing all multicellular animals. Fossil and molecular data place the origin of metazoans in the Precambrian, with diversification in the Paleozoic and later periods. Metazoans include countless species that are central to ecosystems and to scientific research as model organisms.