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Deuterostomia is a major clade of animals that includes echinoderms, hemichordates, and chordates. The name "deuterostome" means "mouth second," referring to the embryonic development where the blastopore, the first opening in the gastrula, develops into the anus, while the mouth forms later. This is in contrast to protostomes, where the blastopore becomes the mouth. This fundamental difference in embryonic development is a key distinguishing feature between these two major groups of bilaterally symmetrical animals.

Echinoderms, such as starfish and sea urchins, are exclusively marine and characterized by radial symmetry in their adult form, although their larval stages are bilaterally symmetrical. Hemichordates, including acorn worms, possess a proboscis, a short coelom, and a pharynx with gill slits, exhibiting some chordate-like features. Chordates, the most familiar group to many, include vertebrates like fish, amphibians, reptiles, birds, and mammals, as well as invertebrates like tunicates and lancelets. A defining characteristic of chordates, present at some stage of their life cycle, are a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail.

The phylogenetic relationships within Deuterostomia are a subject of ongoing research, but molecular data strongly supports their monophyly. The evolutionary history of deuterostomes spans from the Cambrian period, with fossils providing insights into their early diversification and the origins of major lineages. Their ecological roles are diverse, ranging from sessile filter feeders to active predators, playing crucial roles in marine and terrestrial ecosystems.