A clade of marine nonvertebrate chordates in which the body is typically enclosed in a protective tunic of a cellulose-like material and the notochord and dorsal hollow nerve cord are evident only in the free-swimming tadpole-like larval stage. The pharynx is specialized for filter feeding (seeendostyle). There are three classes: the sessile Ascidiacea (sea squirts), the pelagic Thaliacea (salps), and the pelagic Larvacea (in which the larval form is retained into adulthood).
Geology and Earth Sciences
Subphylum comprising the sea-squirts, with reliable fossils from the Permian, and possible remains in Silurian sediments. Sea-squirts have tadpole-like larvae with notochords in their tails, a feature that links them with the chordates.