Deuterostomia

Deuterostomia is a major clade within the animal kingdom (Bilateria) that includes the two largest and most studied lineages of animals: Ambulacraria (echinoderms and hemichordates) and Chordata (including all vertebrates and their relatives). The exact composition of Deuterostomia has been refined with molecular data, and some analyses recognize a few early-branching groups whose placement remains debated, such as Xenacoelomorpha, which is sometimes described as outside or at the base of the deuterostome lineage.

A defining feature of deuterostomes is their embryonic development. During early development, deuterostomes typically exhibit radial and indeterminate cleavage, and the blastopore that forms during gastrulation becomes the anus, with the mouth forming later. Coelom formation usually occurs by enterocoely, in which coelomic compartments bud from the primitive gut. These developmental traits contrast with protostomes, where cleavage is often spiral and determinate, the mouth forms first, and the coelom arises by schizocoely.

Morphology and diversity are broad within Deuterostomia. Chordates encompass a wide range of animals from fish to mammals, including species capable of life on land. Ambulacrarians comprise echinoderms such as sea stars and sea urchins, and hemichordates like acorn worms, many of which are marine. The grouping has been influential in studies of animal evolution and comparative embryology, helping researchers understand the origins of complex organ systems and the evolution of major body plans.