General Physiology, Morphology, and Development
Like other chordates, tunicates have a notochord during their early development, but by the time they have completed their larval stages they have lost all myomeric segmentation throughout the body. As members of the Chordata they are true Coelomata with endoderm, ectoderm and mesoderm, but they do not develop very clear coelomic body-cavities if any at all. Whether they do or not, by the end of their larval development all that remain are the pericardial, renal, and gonadal cavities of the adults. Except for the heart, gonads, and pharynx (or branchial sac), the organs are enclosed in a membrane called an epicardium, which is surrounded by the jelly-like mesenchyme. Tunicates begin life in a mobile larval stage that resembles a tadpole. A minority of species, those in the Larvacea, retain the general larval form throughout life, but most Tunicata very rapidly settle down and attach themselves to a suitable surface, later developing into a barrel-like and usually sedentary adult form. The Thaliacea however, are pelagic throughout their lives. and their life cycles may be complex, as for example in the Salpida.
Tunicates lack kidney-like metanephridial organs, but make do with a less elaborate nitrogenous excretory system. The typical renal organ is a mass of large clear-walled vesicles that occupy the rectal loop, and the structure has no duct. Each vesicle is a remnant of a part of the primitive coelom, and its cells extract nitrogenous waste matter from circulating blood. They accumulate the wastes inside the vesicles as urates and do not have any obvious means of disposing of the material during the life of the animal.
Tunicates are unusual among animals in that they produce a large fraction of their tunic and some other structures in the form of cellulose. Cellulose production in animals is so inconspicuous that some sources deny its presence outside plants, but for some time now it has been known to occur in the dermis of mammals. However, the Tunicata are unique in their scale of application and production of the material. When in 1845 Carl Schmidt first announced the presence in the test of some Ascidians of a substance very similar to cellulose, he called it "tunicine", but it now is recognised as cellulose rather than any alternative substance.
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