Tunicate
A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (/ˌtjuːnɪˈkeɪtə/). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time called Urochordata, and the term urochordates is still sometimes used for these animals. They are the only chordates that have lost their myomeric segmentation, with the possible exception of the 'seriation of the gill slits'.[6][7]
Some tunicates live as solitary individuals, but others replicate by budding and become colonies,[8] each unit being known as a zooid. They are marine filter feeders with a water-filled, sac-like body structure and two tubular openings, known as siphons, through which they draw in and expel water. During their respiration and feeding, they take in water through the incurrent (or inhalant) siphon and expel the filtered water through the excurrent (or exhalant) siphon. Most adult tunicates are sessile, immobile and permanently attached to rocks or other hard surfaces on the ocean floor; others, such as salps, larvaceans, doliolids and pyrosomes, swim in the pelagic zone of the sea as adults.
Various species of the subphylum tunicata are commonly known as ascidians, sea squirts, tunicates, sea pork, sea livers, or sea tulips.
The earliest probable species of tunicate appears in the fossil record in the early Cambrian period. Despite their simple appearance and very different adult form, their close relationship to the vertebrates is evidenced by the fact that during their mobile larval stage, they possess a notochord or stiffening rod and resemble a tadpole. Their name derives from their unique outer covering or "tunic", which is formed from proteins and carbohydrates, and acts as an exoskeleton. In some species, it is thin, translucent, and gelatinous, while in others it is thick, tough, and stiff.
About 3,000 species of tunicate exist in the world's oceans, living mostly in shallow water. The most numerous group is the ascidians; fewer than 100 species of these are found at depths greater than 200 m (660 ft).[9] Some are solitary animals leading a sessile existence attached to the seabed, but others are colonial and a few are pelagic. Some are supported by a stalk, but most are attached directly to a substrate, which may be a rock, shell, coral, seaweed, mangrove root, dock, piling, or ship's hull. They are found in a range of solid or translucent colours and may resemble seeds, grapes, peaches, barrels, or bottles. One of the largest is a stalked sea tulip, Pyura pachydermatina, which can grow to be over 1 metre (3.3 ft) tall.[9]
The Tunicata were established by Jean-Baptiste Lamarck in 1816. In 1881, Francis Maitland Balfour introduced another name for the same group, "Urochorda", to emphasize the affinity of the group to other chordates.[10] No doubt largely because of his influence, various authors supported the term, either as such, or as the slightly older "Urochordata", but this usage is invalid because "Tunicata" has precedence, and grounds for superseding the name never existed. Accordingly, the current (formally correct) trend is to abandon the name Urochorda or Urochordata in favour of the original Tunicata, and the name Tunicata is almost invariably used in modern scientific works.It is accepted as valid by the World Register of Marine Species[11] but not by the Integrated Taxonomic Information System.[12]
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