Tunicates or urochordates (appendicularians, salps and sea squirts) is a sub phylum of group of underwater saclike filter feeders belonging to phylum Chordata. The body of an adult tunicate is quite simple, being essentially a sack with two siphons through which water enters and exits. Water is filtered inside the sack-shaped body. Tunicates exhibit all important characters of chordates like, in having notochord, a dorsal nerve cord, pharyngeal slits, and a post-anal tail. Tunicates begin life in a mobile larval stage that resembles a tadpole.The larval stage ends when the tunicate finds a suitable rock to affix to and cements itself in place.
The larval form is not capable of feeding, and is only a dispersal mechanism. The moment they find a suitable spot on the sea floor and the settle in a head down, tail up position. They attach themselves to the sea floor using special adhesive glands in the front of their head. Once settled they undergo an amazing metamorphosis during which the trademarks of the phylum Chordata, the post-anal tail and the notochord it contained are lost. The remainder of the body twists through 180 degrees in order to become a small tunicate. Most tunicates are thought to live about one year as adults. Many physical changes occur to the tunicate’s body, one of the most interesting being the digestion of the cerebral ganglion, which controls movement and is the equivalent of the human brain in some arthropods and insects.
Its not that all tunicates spend thier life at the ocean floor ,in fact some species are seen on the surface of water as well . These tunicates which are entirely pelagic, typically have barrel-shaped bodies and may be extremely abundant in the open ocean.The Class Appendicularia (also known as Larvacea) includes a variety of mostly inconspicuous, small pelagic tunicates. eg : Oikopleura species. The salps (Class Thaliacea, Order Salpida) include the most commonly encountered pelagic tunicates and other groups of thaliaceans include the doliolids (Order Doliolida) and pyrosomes (Order Pyrosomatida).
Tunicates are mostly hermaphroditic, meaning that have both male and female reproductive organs, but self fertilisation is often avoided by either having the eggs and sperm chemically designed to reject each other, or by varying the maturation time of egg and sperms. Sperm are released into the sea but the eggs are retained within the body where they are fertilised by sperm brought in with incoming water.
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.
Sea squirts are more closely related to vertebrates (including fish, birds, and humans) than worms, sea stars, or other invertebrates.
The Tunicata contains about 3,000 species, traditionally divided into the following classes:
* Ascidiacea (Aplousobranchia, Phlebobranchia, and Stolidobranchia)
* Thaliacea (Pyrosomida, Doliolida, and Salpida)
* Appendicularia (Larvacea)