Ambulacraria

Ambulacraria is a superphylum of the deuterostomes that includes the echinoderms (such as starfish, sea urchins, and sea cucumbers) and the hemichordates (enteropneusts, commonly known as acorn worms, and the colonial pterobranchs). The name derives from ambulacral structures of echinoderms, notably the tube feet and associated canals used in locomotion and feeding, reflecting a shared evolutionary heritage with hemichordates.

Echinoderms are exclusively marine and possess a calcareous endoskeleton and a characteristic pentaradial symmetry in the adult form, though larvae are bilaterally symmetric. They have a water vascular system with tube feet, enabling locomotion, feeding, and respiration. Echinoderms display a wide range of forms, including starfish, sea urchins, brittle stars, sea cucumbers, and crinoids.

Hemichordates comprise the enteropneusts (acorn worms) and the pterobranchs. They typically exhibit a tripartite body plan with a proboscis, a short collar, and a trunk, and they bear pharyngeal gill slits and a dorsally situated nerve cord. Hemichordates are marine and can be wormlike or colonial, with pterobranchs often living in tubes.

Ambulacraria is supported by comparative embryology and molecular phylogenetics as a clade within Deuterostomia, generally regarded as the sister group to the Chordata. The group helps illuminate early deuterostome evolution, including the diversification of feeding and locomotory systems, and the origin of the echinoderm water vascular apparatus.