|Lamarck , 1816|
The tunicates (Tunicata, Urochordata) are a purely marin common world subtype of chordates . They live as sessile animals on the sea floor ( sea squirts ) or planktonic ( salps , appendicularia ). Tunicates can appear as solitary animals or as colonies . Its name is derived from a coat ( tunica ) made of tunicin (cellulose).
An important feature is a cuticular coat , which is deposited by the single-layer epidermis and - unique in the animal kingdom - consists of the polysaccharide cellulose. A special feature of the tunicata, which is otherwise mainly known from insects , is that its heart has the ability to change the direction of blood flow within a very short time. After the blood from the heart for some time in the gills intestine was driven, the heart stops for a short time and changes the direction of pumping of the blood, so that the stomach is supplied.
Their affiliation to the chordata arises from the organization of the larvae, in which the oar tail is traversed by a chorda dorsalis and a neural tube . Only in the pelagic appendixes does this also apply to the adult form. Otherwise they show no myomeric segmentation (V-shaped muscle segments) and no tail structures in the adult stage . They also lack kidney-like nephridia , which means that excretion takes place via the intestinal surface or via storage cells. The blood system is open (lacunar). Characteristic is an inlet and outlet opening for breathing water and plankton food. The pumping direction of the heart is reversed at regular intervals.
Tunicates are microphage filter feeders . The gill intestine of the tunicata has numerous crevices that work like a sieve. A stream of water is generated by means of cilia in the mouth region, which drives food particles through the trap-like gill basket. At the base of the gill intestine lies the hypobranchial groove, the so-called endostyle . It produces a mucus that actually rolls up the swirled-in food particles and ultimately feeds it to the digestive part of the intestine. In adult salps, the peribranchial space opens towards an outflow opening. The openings of the rectum and the genital openings into a cloacal space are also found here. Because of its ability to absorb iodine, the hypobranchial trough can be placed in a developmental line with the thyroid gland of vertebrates.
The complicated reproductive relationships of some classes of tunicates are also very noticeable: there can be a change between the sexual and asexual generation ( salps ).
In general, asexual reproduction is widespread. It often leads to the formation of characteristic colonies if the daughters stay together. The salps form chains, the fire rollers are known exclusively as colonies, and the sea squirts also develop z. T. sticks or colonies of typical shape.
Traditionally, tunicates are divided into three subtaxa in the rank of classes:
- Appendicularia (Larvacea)
- Sea squirts (Ascidiacea)
- Salps (Thaliacea)
Of these three classes, the sea squirts do not form a monophylum according to DNA sequence analyzes . A cladogram shows the actual relationship:
- Rüdiger Wehner, Walter Gehring: Zoology. 24th edition. Thieme Verlag, Stuttgart 2007, ISBN 978-3-13-367424-9 , p. 787 ff.
- ↑ Entry on Tunicin. In: Römpp Online . Georg Thieme Verlag, accessed on July 6, 2015.
- ↑ Handbook of Experimental Pharmacology. Volume 26: Comparative pharmacology of carrier substances in an animal systematic representation. Springer, Berlin 1971, ISBN 3-540-05132-5 .