Urochordata (sometimes known as tunicata and commonly called urochordates, tunicates or sea squirts) is the subphylum of saclike filter feeders with input and output siphons. They are members of the phylum Chordata, which also includes birds, fish, and mammals. As with other chordates, tunicates possess a notochord during their early stages of development. Larval stages may have the appearance of a tadpole, whereas the adult stage has a much more barrel-like shape. They feed by filtering sea water through a gill basket.

Tunicates consist of two openings in their body cavity. There consists an incurrent as well as excurrent siphon. The incurrent siphon is used for food and water to enter in and the excurrent siphon allows for water as well as waste to pass through and exit the tunicate. The main source of food that the tunicate consumes 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.

Some larval forms appear very much like primitive chordates or hemichordates with a notochord (primitive spinal cord). Some forms have a calcereous spicule that may be preserved as a fossil. Jurassic to Present with one proposed Neoproterozoic form - Yarnemia.

Once grown, adults can develop a covering to protect themselves from enemies.

Tunicate blood is particularly interesting. It contains high concentrations of rare metal vanadium and vanadium-associated proteins. Some Tunicates can concentrate vanadium up to a level one million times that of the surrounding seawater. It is still unknown how they do this or why.

References

• Dennis. 2003. pers. comm. Marine Science Dept. Orange Coast College.
• Solomon, E., L. Berg, D. Martin. 2002. Biology. Brooks/Cole.

External links

• washington.edu page on tunicates (http://depts.washington.edu/fhlk12/StudentProjects/Tun.biology.html)