Subphylum Urochordata (Tunicates)

Subphylum Urochordata (Tunicates)

Tunicates (also called sea squirts, urochordates) are marine animals that, as sessile, saclike adults, look nothing like typical chordates — the adult tunicate has no visible notochord, no post-anal tail, and its nerve cord is reduced to a small ganglion, so at first glance an adult sea squirt does not obviously belong in phylum Chordata at all.

The chordate identity of tunicates is established by their larval stage: the free-swimming tunicate larva (sometimes called a "tadpole larva" for its superficial resemblance to a frog tadpole) clearly shows all four chordate hallmarks — a notochord (confined to the tail region), a dorsal tubular nerve cord, pharyngeal gill slits, and a post-anal tail. When this larva metamorphoses into the sessile, saclike adult form, most of these larval chordate structures (notochord, tail, much of the nerve cord) are lost or drastically reduced, while the pharyngeal gill slits are retained and enlarged into a basket-like pharynx used in filter feeding. …