Eumetazoa

Eumetazoa is a major clade within the animal kingdom that includes all animals with true tissues, distinguishing them from the more basal, tissue-less groups such as the sponges (Porifera). It also excludes placozoans, which lack the organized tissue structure characteristic of eumetazoans.

Members of Eumetazoa possess differentiated tissues derived from embryonic germ layers. The clade comprises two broad lineages: Radiata, which includes Cnidaria (jellyfish, corals) and Cctenophora (comb jellies), and Bilateria, which includes most other animals such as arthropods, molluscs, and chordates. Among Radiata, diploblastic organization (two germ layers) is common, whereas Bilateria is typically triploblastic (three germ layers) with more complex organ systems, though tissue-level complexity is not uniform across all lineages.

Phylogenetically, Eumetazoa represents the lineage of animals with true tissue organization. Within it, Radiata and Bilateria are recognized as major subgroups, reflecting differences in symmetry and body plan, from radial to bilateral and more derived morphologies.

Evolutionary context and fossil record show that eumetazoans arose in the late Precambrian. The appearance of true tissues enabled substantial diversification, with many phyla and body plans appearing during the Cambrian period. Modern eumetazoans encompass the vast majority of animal diversity, including most familiar invertebrate groups and all vertebrate lineages.