eumetazoans

Eumetazoa is a major clade within the animal kingdom that includes all animals with true tissues and a definite body plan, excluding the more primitive forms such as sponges (Porifera) and placozoans. The group comprises the majority of living animals, including the phyla Cnidaria, Ctenophora, and the Bilateria.

Defining features: true tissues organized into germ layers; cnidarians and ctenophores are diploblastic with two germ layers, while most eumetazoans are triploblastic with three germ layers (ectoderm, endoderm, and mesoderm). They typically exhibit organized symmetry (radial in cnidarians, bilateral in most others) and possess a nervous system, a gut or digestive compartment, and various organ systems. Development commonly involves embryonic cleavage and gastrulation that establish tissues and organs.

Major groups: Cnidaria (jellyfish, corals, sea anemones), Ctenophora (comb jellies), and Bilateria, which encompasses most other animals, including protostomes (arthropods, mollusks, annelids) and deuterostomes (chordates, echinoderms).

Evolution and diversity: Eumetazoans arose early in animal evolution and display a wide range of body plans, from sessile to highly mobile, with various levels of organization. Sponges and placozoans lie outside this clade, representing more primitive lineages.