The urochordés , also called tunicates or tunicates , are a subphylum of the Chordés. They count approximately 1500 species.

Others twisted: Céphalocordés (20 sp) + Craniates (45.000 sp).

Pro-twisted are formed by the groups of Urocordés and the Céphalocordés. This Phylogénie goes back to approximately 530 MY. Recent elements tend to prove that urocordés closer to vertebrate than are céphalocordés.

Organization of urocordés

Characters

• Absence of head in the adult (fixed life).
• Chorde (or notochorde) presents only in the posterior part of the animal (definition of the urochordés ).
• biological Cycle marked by the succession of a larval stage nectonic (swimming freely) and of an adult stage sessile filterer.
• Body of the adult surrounded by a tunic (definition of the tunicier ), presenting of 2 siphons (oral and cloacal), bored pharynx of slits branchiales leading in the cavity péribranchiale.
• According to the assumption of Garstang, the larva became during the evolution the stage dominating in the vital cycle, and all the

descendants have the characteristics of the larva of Urochordés (Néoténie): a chorde, slits pharyngiennes, an post-anal tail, and a dorsal nervous tube.

At the larval stage, the characters of the Chordé S are present:

• a notochorde, which ensures the rigidity of the tail.

• a dorsal tube neural with sensory function (photosensitive beach and a cell statoréceptrice) and motor coach (ganglion known as " viscéral" innervant the muscles of the tail).
• a ventral pharynx (which, at many species develops only after the metamorphosis).

Phylogeny

3 classes:

• Ascidies .

The larva fixes itself irréversiblement on a solid support and undergoes a radical metamorphosis: the cord, the tail and the tube neural disappear. The pharynx or bag branchial hypertrophies and ensures the nutrition and the breathing of the animal by filtering water: this one returns in the pharynx by the oral siphon, crosses its wall by multiple slits branchiales (or " stigmates"), passes in a second cavity surrounding the pharynx (the atrium) to be finalment expelled by the siphon atrial. The gaseous exchange is made with the duct drainage in the marks, while the nutritive particles are intercepted by a mucus filter papering the wall internal of the pharynx, and led towards the esophagus, the stomach and the rectum. The rectum also opens in the atrium and deposit them are expelled with the water current by the siphon atrial. Ectoderm secret a tunic made up of Tunicine (polysaccharide close to the Cellulose). The tunic grows with the animal.

• Thaliacé S (Salpe S, dolioles and pyrosomes).

They are organizations pelagic S in the shape of barrel. The reproductive cycle consists of an alternation of sexuées phases (hermaphrodites) and phases asexual. The larvae always do not have a cord.

• Appendicular S or larvacés.

The same plan of larval organization is maintained in the adult (Néoténie).

Internal bonds

• Urochordata (phylogenetic classification)
• taxinomic Photo-guide of the animal world
• taxinomic Photo-guide of fauna and the underwater flora

External bonds

Be-X-old: Абалоньнікі

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