The term Urochordata is derived from the Greek word oura meaning tail and chorda meaning chord. Animals which in the larval stage have a notochord in the tail and the body is covered by a test or tunic are called Urochordata. These are tail cordata. These are commonly called tunicates. So named because their bodies are covered with cellulose called tunic or testa. Some species of this subspecies are called sea fountains because they forcefully siphon water. Tunicates feed on suspension feeders (small food). Scientist Lamarck named them Tunicata. Their number of species is 2,804.
Characteristics of Urochordata
1. A nortochord is absent in mutants. The larval stage has a notochord (in the tail).
2. The body is covered by a tunic or test. Tunic is made up of cellulose called Tunisian.
3. The metamorphosis adheres to submerged objects by adhesive glands located on the head. That is, living a still life.
4. They are marine and live singly or in colonies.
5. Some animals forcefully draw water with a siphon, so they are called sea fountains or tunicates.
6. The larval stage consists of notochord, nerve cord and phloem. The larva is the free or active stage.
7. The pharynx is like a large sac.
8. Their bodies contain chemicals called didemnin. Didemenin is used in the treatment of cancer.
9. The circulatory system is free, the heart cycle reverses peristalsis (blood flow reverses direction each time).
10. They are bisexual animals. The life cycle consists of tadpole larvae.
11. Larval metamorphosis or retrogressive metamorphosis occurs. That is, advanced features are lost and undeveloped features are gained.
[Key words to remember features: notochord, tunic, testa, fountain, nerve cord, pharynx, pharyngeal sac, didemnin, reverse peristalsis, tadpole, retrogressive metamorphosis]
Classification of Eurochordata
Eurochordata are divided into three classes. The classes are Ascidaceae, Larvaceae and Thalaceae.
1. Ascidiacea: The number of species in Ascidiacea class is 2,860.
(i) They are brightly colored marine animals. The shape of the body is cylindrical or sac-like.
(ii) They are solitary or colonial, stationary or free moving tunicates. Members of the colony are connected to each other by stolons.
(iii) They are known as ascidians, tunicates and marine squits.
(iv) They are called sea fountains.
(v) The body is covered by a soft and leathery tunic.
(vi) The free end of the body consists of external and internal siphons.
(vii) Has numerous pharyngeal florets. The alimentary canal is T-shaped.
(viii) A metamorphosis does not have a tail.
(ix) The notochord disappears in the mature state. They live in shallow water.
Examples: Ascidia mentula, Molgula oculata, Ciona intestinalis, Herdmania momus.
2. Larvacea: Larvacea class has 70 species.
(i) They are small, elongated and much curved frog-shaped animals.
(ii) Complete animals have tail and notochord. The tail is thin and flat.
(iii) Pelagic animals live at the top of the water and benthic animals live at the bottom.
(iv) They have notochord, dorsal nerve cord and smooth muscle in their tail.
(v) They have two gill openings in the pharynx. However, atriopore does not exist.
(vi) Body covering is thin, transparent and temporary.
(vii) Body siphon absent.
(viii) Life cycle of tadpole larvae.
Examples: Oikopleura dioica, Bathochordaeus stygius, Appendicularia sicula.
3. Thaliacea: Number of species of Thaliacea class is 70.
(i) They are solitary or colonial and free floating tunicates.
(ii) Their bodies are lemon or barrel shaped. In the mature state, the body does not have a tail.
(iii) Their body is covered by a thin and transparent tunic.
(iv) Body wall consists of circular muscle rings.
(v) present at the opposite ends of the orifice and atriopo.
(vi) Body has external siphon at one end and internal siphon at other end.
(vii) Many have light-scattering luminous organs in their bodies. Luminous organ emits bright light at night.
(viii) Polymorphism is seen among them. There is no tail.
Examples: Doliolum rarum, Salpa maxima, Pyrosoma atlanticum.