eumetazoan

An eumetazoan is any animal within Eumetazoa, a major lineage of Metazoa defined by the possession of true tissues arranged into germ layers during development. This contrasts with the Porifera (sponges) and some other basal metazoans, which lack true tissues.

Eumetazoans include both diploblastic groups, such as Cnidaria (jellyfish, corals, sea anemones) and Ctenophora (comb jellies), and triploblastic groups, collectively the Bilateria (arthropods, mollusks, annelids, chordates, and many others).

Their tissues form through germ-layer–based development, typically with an ectoderm and endoderm (and a mesoderm in most bilaterians). Many have simple or complex organ systems and a nervous system, often with a gut or gastrovascular cavity.

Evolutionarily, eumetazoans appear in the late Precambrian, with Cambrian diversification reflecting widespread body plans and feeding strategies. Fossil evidence and molecular data support Eumetazoa as a well-supported clade within Metazoa.

Taxonomically, eumetazoans represent a broad, defining branch; their study helps distinguish true tissue-grade organization from the more primitive organization seen in sponges and placozoans.