Urochordates have four fundamental characteristics of chordates in the larval stage but adult urochordates possess neither a notochord nor a dorsal nerve cord. The tadpole-like larvae are planktonic and possess the fundamental chordate characteristics that help them in finding a substratum for attachment. Body of Ascidians is covered by a tunic or test made of cellulose-like polysaccharide, tunicin.
Urochordata are a medium sized group of around 3,000 species of marine animals commonly referred to as Sea Squirts, Tunicates, Salps and Larvaceans. They are all filter feeders using a basically similar mechanism of drawing water through perforated pharynx which collects small food particles trapped in mucous.
In some cases this matrix of the test contains living cells that have migrated from the main body of the animal and a network of blood vessels. The larvacea can change their house and secrete a new house every four hours or so. Urochordate adults have no limbs, no brain and except in the larvaceans the tail is only evident during larval development.
Tunicates are also known to contain useful chemical compounds, such as, Didemnins,Esteinascidin and Aplidine, which are effective as anticancer, antivirals and immunosuppressants.
The best known fossil urochordate is Shankouclavashankouense from the Lower Cambrian Maotianshan Shale at Shankou village near Kunming (South China). Another tunicate fossil of Catellocaulavallata was found in upper Ordovician rocks in the midwestern United States.
Class – Ascidiacea (Aplousobranchia, Phlebobranchia, and Stolidobranchia).
There are 4 orders in Ascidiacea. Examples: Ascidia, Botryllus, Chelyosoma, Ciona, Clavelina, Corella, Diazona, Diplosoma, Halocynthia, Lissoclinum, Molgula, Polycarpa, Psammascidia, Pyura, Styela.
The branchial and atrial siphons are close together on the top, away from the point of attachment. The water filters through a large pharyngeal basket where the gill slits filter out suspended organic particles and transfer them to the oesophageal opening through the dorsal lamina. Once attached, the animal does not move and covers itself with a protective tunic by secreting a thick substance similar to cellulose. Some species are colonial and reproduce by budding. They earn the name sea squirt by suddenly squirting out water by contraction of muscles when disturbed.
Class – Thaliacea. 3 orders (Pyrosomida, Doliolida, and Salpida). Examples: Dolioletta, Doliolum, Pyrosoma, Salpa, Thetys.
They are free-living planktonic tunicates that are barrel shaped, with the incurrent siphon and the excurrent siphon on the opposite ends. The adults swim about in water in search of plankton on which it feeds. Sometimes they can achieve astonishing concentrations of thousands of animals per cubic meter of water. Because the branchial and atrial siphons are located at opposite ends of the barrel-shaped, jet like exhalant water current provides propulsive force for fast movement. They show alternation of solitary asexual and sexual forms.
Class – Larvacea (Appendicularia). Examples: Fritillaria, Oikopleura, Stegasoma.
Only one order in this class.
They are called Larvacea because they retain larval stage throughout their lives and develop gonads. The adult animals have a relatively large tail that is used to draw current of water into a gelatinous house with a sophisticated set of filters and openings inside which the animal lives. The animal consumes small particles that are brought into the house and abandons the house when its filters are clogged with debris and constructs a new house. Pharynx has only 2 stigmata that open directly to the body surface and endostyle is reduced; tail with notochord and nerve cord is much larger than body.
Class – Sorberacea. Example: Octacnemus.
They are ascidian-like animals of deep ocean. Little is known of them except that they have the same life history as the ascidians. They retain a dorsal nerve chord in the adult stage and do not have gill slits in the pharynx. Brusca and Brusca (2003) state that they are carnivorous.