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Tunicate
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===Body structure=== By far the largest class of tunicates is the [[Ascidiacea]]. The body of an ascidiacean is surrounded by a [[Test (biology)|test]] or tunic, from which the [[subphylum]] derives its name. This varies in thickness between species but may be tough, resembling cartilage, thin and delicate, or transparent and gelatinous. The tunic is composed of proteins, crosslinked by phenoloxidase reaction,<ref>{{Cite journal |last1=Daugavet |first1=M. A. |last2=Dobrynina |first2=M. I. |last3=Shaposhnikova |first3=T. G. |last4=Solovyeva |first4=A. I. |last5=Mittenberg |first5=A. G. |last6=Shabelnikov |first6=S. V. |last7=Babkina |first7=I. Yu |last8=Grinchenko |first8=A. V. |last9=Ilyaskina |first9=D. V. |last10=Podgornaya |first10=O. I. |date=2022-08-22 |title=New putative phenol oxidase in ascidian blood cells |journal=Scientific Reports |language=en |volume=12 |issue=1 |pages=14326 |doi=10.1038/s41598-022-18283-9 |issn=2045-2322 |pmc=9395347 |pmid=35995990 |bibcode=2022NatSR..1214326D }}</ref> and complex carbohydrates, and includes [[tunicin]], a variety of cellulose. The tunic is unique among invertebrate exoskeletons in that it can grow as the animal enlarges and does not need to be periodically shed. Inside the tunic is the body wall or mantle composed of [[connective tissue]], [[muscle]] fibres, [[blood vessel]]s, and [[nerve]]s. Two openings are found in the body wall: the buccal siphon at the top through which water flows into the interior, and the atrial siphon on the ventral side through which it is expelled. A large pharynx occupies most of the interior of the body. It is a muscular tube linking the buccal opening with the rest of the gut. It has a ciliated groove known as an [[endostyle]] on its ventral surface, and this secretes a mucous net which collects food particles and is wound up on the dorsal side of the pharynx. The gullet, at the lower end of the pharynx, links it to a loop of gut which terminates near the atrial siphon. The walls of the pharynx are perforated by several bands of slits, known as stigmata, through which water escapes into the surrounding water-filled cavity, the atrium. This is criss-crossed by various rope-like [[Mesentery (zoology)|mesenteries]] which extend from the mantle and provide support for the pharynx, preventing it from collapsing, and also hold up the other organs.<ref name=Ruppert/> The [[Thaliacea]], the other main class of tunicates, is characterised by free-swimming, pelagic individuals. They are all filter feeders using a pharyngeal mucous net to catch their prey. The [[pyrosome]]s are [[Bioluminescence|bioluminous]] colonial tunicates with a hollow cylindrical structure. The buccal siphons are on the outside and the atrial siphons inside. About ten species are known, and all are found in the tropics. The 23 species of [[Doliolida|doliolids]] are small, mostly under {{convert|2|cm|abbr=on}} long. They are solitary, have the two siphons at opposite ends of their barrel-shaped bodies, and swim by jet propulsion. The 40 species of [[Salpida|salps]] are also small, under {{convert|4|cm|abbr=on}} long, and found in the surface waters of both warm and cold seas. They also move by jet propulsion, and often form long chains by budding off new individuals.<ref name=Ruppert/> A third class, the [[Larvacea]] (or Appendicularia), is the only group of tunicates to retain their chordate characteristics in the adult state, a product of extensive [[neoteny]]. The 70 species of larvaceans superficially resemble the tadpole larvae of amphibians, although the tail is at right angles to the body. The [[notochord]] is retained, and the animals, mostly under 1 cm long, are propelled by undulations of the tail. They secrete an external mucous net known as a house, which may completely surround them and is very efficient at trapping planktonic particles.<ref name=Ruppert/>
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