Tunicate, also known as urochordata, tunicata (and by the common names of urochordates, sea squirts, and sea pork[1]) is the subphylum of a group of underwater saclike filter feeders with incurrent and excurrent siphons, that are members of the phylum Chordata. Most tunicates feed by filtering sea water through pharyngeal slits, but some are sub-marine predators such as the Megalodicopia hians. Like other chordates, tunicates have a notochord during their early development, but lack myomeric segmentation throughout the body and tail as adults. Tunicates lack the kidney-like metanephridial organs, and the original coelom body-cavity develops into a pericardial cavity and gonads. Except for the pharynx, heart and gonads, the organs are enclosed in a membrane called an epicardium, which is surrounded by the jelly-like mesenchyme. Tunicates begin life in a mobile larval stages that resembles a a tadpole, later developing into a barrel-like, sedentary adult form.

Tunicates are suspension feeders. They have two openings in their body cavity: an in-current and an ex-current siphon. The in-current siphon is used to intake food and water, and the ex-current siphon expels waste and water. The tunicate's primary food source is plankton. Plankton gets entangled in the mucus secreted from the endostyle. The tunicate's pharynx is covered by miniature hairs called ciliated cells which allow the consumed plankton to pass down through to the esophagus. Their guts are U-shaped, and their anuses empty directly to the outside environment. Tunicates are also the only animals able to create cellulose.

Tunicate blood is particularly interesting. It contains high concentrations of the transition metal vanadium and vanadium-associated proteins. Some Tunicates can concentrate vanadium up to a level one million times that of the surrounding seawater. Specialized cells can concentrate heavy metals, which are then deposited in the tunic.

Sea squirts are more closely related to fish, birds, and humans than worms, sea stars, or other invertebrates.

The Tunicata contains about 3,000 species, usually divided into the following classes:

Ascidiacea, (Aplousobranchia, Phlebobranchia, and Stolidobranchia)
Thaliacea
Appendicularia (Larvacea)
Sorberacea.
Although the traditional classification is followed for now, newer evidence suggests that the Ascidiacea is an artificial group. The new classification would be:

Stolidobranchia,
Phlebobranchia and Thaliacea,
Aplousobranchia and Appendicularia,
Sorberacea would belong somewhere in Ascidiacea, or be in a taxon on its own.
The species Ciona intestinalis has attracted interest in biology for developmental studies.