The tunicates were first regarded as sponges. Lamark in 1816 placed Tunicata in between the Radiata and Vermes in his system of classification. Later, they were included in Mollusca. In 1866 Kowalevsky kept them in chordates.
Their chordate features are clearly seen in the larval stages
. All Urochordates are marine and occur in all the seas. Majority of them are sedentary and some are pelagic.
- Body shows variation in size and form.
- The body is un segmented and has no tail.
- The body is covered by a test. It is formed by tunicine which is rallied to cellulose. Hence the name Tunicata.
- Body wall shows one-layered epidermis, dermis is made by connective tissue and muscles, and atrial epithehum.
- Celome is absent.
- Atrial cavity surrounds the pharynx, into this cavity the gill slits,anus and genital ducts will open. It opens through atrial aperture.
- Larva has notochord in the tail. It disappears during metamorphosis.
- Respiratory system contains gills in the pharyngeal wall.
- Ciliary mode of feeding is common.
- Open type of Circulatory system is seen.
- The heart is ventral and it periodically reverses its function.
- Nervous system is represented by a single dorsal ganglion in the adult.
- Excretion is carried on by nephrocytes.
- Asexual reproduction is by budding.
- Bisexual animal and cross fertilisation is favored.
- Fertilization is external.
- Development includes a minute, free swimming tadpole larva with a tail, a dorsal nerve cord, and a notochord in the tail. In some urochordates retrogressive metamorphosis is seen in the life history.