deuterostomes

Deuterostomia is a superphylum within the Animal kingdom that groups together several major lineages of bilaterian animals, most notably the chordates (which include vertebrates) and the echinoderms (such as stars and sea urchins), along with the hemichordates (acorn worms). Traditionally, Deuterostomia was defined by shared developmental features and is often contrasted with the protostomes, which include most invertebrates like arthropods and mollusks. Some modern classifications debate the exact composition, and Xenacoelomorpha is variably placed in discussions of early deuterostome relationships.

A defining feature of deuterostomes is their embryonic development. They typically exhibit radial and indeterminate cleavage, meaning embryonic cells retain the capacity to develop into a complete organism even when separated. The blastopore, the first opening of the developing gut, becomes the anus, while the mouth forms later from another region. The coelom, the body cavity, often forms by enterocoely, an outpocketing of the gut.

Morphologically, deuterostomes are triploblastic and coelomate, with body plans ranging from the elongated, segmented chordates to the often more centralized echinoderms whose adults may display radial symmetry. Echinoderms have a water vascular system and a characteristic larval bilateral symmetry, while chordates possess features such as a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail at some life stage. Hemichordates share some chordate-like features, such as pharyngeal slits, but lack a true vertebrate-type dorsal nerve cord.

The group includes many well-known animals, from sea urchins and starfish to fishes, reptiles, birds, and mammals. Deuterostome development is a central theme in evolutionary and developmental biology, highlighting contrasts with protostome development.