The Tunicata (or Urochordata) are hermaphrodite marine chordate animals, contained in secreted tunic in adult stage, which are usually sessile and degenerate, and may be solitary or colonial, fixed or free; the chordate characters in these are well represented in their larval stage in which the notochord is restricted to the posterior part of the body (tail).
The Urochordata (Gr. uro, tail) are marine animals found practically in all parts of the sea, and at all depths. They extend from the Arctic and Antarctic regions to the tropical waters and from the littoral zone down to the’abyssal depth of over three miles. In these peculiar ciliary-feeding animals the chordate character is lost in the adult. For this reason they had varied history. They were included with the invertebrates such as the Polyzoa and Brachiopodaby early writers.
Historically speaking the ascidians are quite old known animals. Aristotle was the first to describe a simple ascidian calling it Tethyum more than two thousand years ago.
The real advance in the study of this group, however, was made after the description of Botryllus1 a compound ascidian. On the basis of the study of Cuvier(1815) on simple ascidians and Savigny (1815) on compound ascidians. Lamarck (1816) crcatcd the class Tunicata, including, besides the simple and compound ascidians, the pelagic forms Pyrosomu and Salpa. Carl Schmidt (1845) discovered the presence of “tunicin” a substance similar to cellulose. Others who contributed to the study of ascidians include Huxley (1851), Gegenbaur, Vogt, H. Muller, Krohn, Leuckart. H. Milne, Edwards. Hcrdman. Mctcalf, Minot, Bateson, Garstangand recently brien( 1930)and Berrill (1951), etc.
Ascidians were formerly regarded a degenerate offshoot of the vertebrates but now they are regarded as primitive stock from which the vertebrates have arisen. If the ascidians represent the stock from which the vertebrates and other chordatcs evolved, the events we arc concerned with occurred no later than Cambrian times and in all probability at a much earlier period. Accordingly, the original ascidian stock has had 500 million or more in which to evolve, differentiate, and increase in size, all the time retaining the ascidian tadpole as a larval form. During such an eternity, which embraces the entire period of vertebrate evolution from the formation of the first free-swimming adult chordate to the present remarkable exhibition of vertebrate behaviour, the ascidians as a group have had ample opportunity or acquire specialization which confuse the picture” (Berrill).