hemichordate

Hemichordata is a phylum of small marine deuterostome animals that include two living lineages, the Enteropneusta (acorn worms) and the Pterobranchia. Together with echinoderms, they are grouped in the clade Ambulacraria, and their features shed light on the evolution of chordates.

Hemichordates display a tripartite body plan consisting of a proboscis, a collar, and a trunk. They bear pharyngeal slits and a stomochord, a duct-like structure linking the pharynx to the foregut; the stomochord is not homologous to the vertebrate notochord. They possess a simple nervous system with a nerve net and, in the collar region, a dorsal nerve cord. Most species respire and circulate via a combination of diffusion across the body surface and an open circulatory system.

Ecology and reproduction: Enteropneusts are mobile burrowers and deposit feeders; pterobranchs are small colonial tube-dwellers. Most hemichordates are marine and occur worldwide, from shallow to deep waters. Reproduction is typically sexual, with direct or indirect development; many pterobranchs brood embryos within colonies. A free-swimming tornaria larva is known in some groups, illustrating deuterostome affinities and similarity to echinoderm larvae.

Fossil record and significance: Hemichordates have a fossil record including graptolites, which are important for stratigraphy. Modern hemichordates provide crucial information about early deuterostome evolution and the origin of chordates.