Urochordata

Urochordata, or tunicates, is a subphylum of the phylum Chordata that includes about 3,000 described species. It comprises three extant classes: Ascidiacea (sea squirts), Thaliacea (salps and doliolids), and Appendicularia (larvaceans). Most adult tunicates are sessile and filter feeders, though thaliaceans are pelagic. Larvae resemble tadpoles and possess chordate features such as a notochord and dorsal nerve cord, but adults often lose these traits.

Morphology and life cycle: The body of most tunicates is enclosed by a tunic, a cellulose-like outer covering called tunicin. They have a pharyngeal basket with slits, and incurrent and excurrent siphons used to draw in water and filter feed. Adults typically lack many vertebrate features; only the larval stage shows the defining chordate characters, including a notochord.

Reproduction and development: Many species are hermaphroditic; fertilization can be external or internal. Embryos develop into free-swimming, tadpole-like larvae which metamorphose into sessile adults. Some ascidians reproduce asexually by budding, and colonial species can form large structures from multiple individuals.

Ecology and significance: Tunicates are widespread in marine environments, from shallow coastal zones to the deep sea. They play important roles as filter feeders and can act as fouling organisms on ships and infrastructure. They are of interest in evolutionary biology because they are the closest living relatives of vertebrates, and model organisms such as Ciona intestinalis and Ciona robusta have been sequenced to study chordate development and genetics.