Editing Tunicate (section)

===Physiology and internal anatomy===
[[File:Uroc005b.png|thumb|400px|right|{{center|Internal anatomy of a generalised tunicate}}]]
[[File:Pyrosoma 001.png|400px|thumb|Section through the wall of an ascidian pyrosoma showing several zooids; (br)&nbsp;buccal siphon; (at)&nbsp;atrial siphon; (tp)&nbsp;test process; {{nobr|(br s) pharynx.}}]]

Like all other [[chordate]]s, tunicates have a [[notochord]] during their early development, but it is lost by the time they have completed their metamorphosis. As members of the Chordata, they are true [[Body cavity|Coelomata]] with [[endoderm]], [[ectoderm]], and [[mesoderm]], but they do not develop very clear [[coelom]]ic body cavities, if any at all. Whether they do or not, by the end of their larval development, all that remain are the [[pericardial cavity|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]].

Ascidian tunicates begin life as a lecithotrophic (non-feeding) mobile [[larva]] that resembles a tadpole,<ref>{{cite journal |url=https://www.sciencedirect.com/science/article/pii/S0012160608013766 |doi=10.1016/j.ydbio.2008.11.026 |title=Delineating metamorphic pathways in the ascidian Ciona intestinalis |year=2009 |last1=Nakayama-Ishimura |first1=Akie |last2=Chambon |first2=Jean-Phillippe |last3=Horie |first3=Takeo |last4=Satoh |first4=Nori |last5=Sasakura |first5=Yasunori |journal=Developmental Biology |volume=326 |issue=2 |pages=357–367 |pmid=19100250 |url-access=subscription }}</ref> with the exception of some members of the families Styelidae and Molgulidae which has direct development.<ref>{{cite book |chapter-url=https://www.taylorfrancis.com/chapters/edit/10.1201/9781003077992-16/ascidians-renganathan |doi=10.1201/9781003077992-16 |chapter=Ascidians |title=Fouling Organisms of the Indian Ocean |year=2020 |last1=Renganathan |first1=T.K. |pages=507–534 |isbn=9781003077992 |s2cid=241318821 }}</ref> The latter also have several species with tail-less larval forms.<ref>{{cite journal |doi=10.1098/rsob.150053 |title=Tunicates: Exploring the sea shores and roaming the open ocean. A tribute to Thomas Huxley |year=2015 |last1=Lemaire |first1=Patrick |last2=Piette |first2=Jacques |journal=Open Biology |volume=5 |issue=6 |page=150053 |pmid=26085517 |pmc=4632506 }}</ref><ref>{{cite journal |pmid=33881514 |year=2021 |author1=Fodor ACA |last2=Powers |first2=M. M. |last3=Andrykovich |first3=K. |last4=Liu |first4=J. |last5=Lowe |first5=E. K. |last6=Brown |first6=C. T. |last7=Di Gregorio |first7=A. |last8=Stolfi |first8=A. |last9=Swalla |first9=B. J. |title=The Degenerate Tale of Ascidian Tails |journal=Integrative and Comparative Biology |volume=61 |issue=2 |pages=358–369 |doi=10.1093/icb/icab022 |pmc=10452958 |doi-access=free }}</ref> The ascidian larvae very rapidly settle down and attach themselves to a suitable surface, later developing into a barrel-like and usually sedentary adult form. The species in the class [[Larvacea|Appendicularia]] are [[Pelagic zone|pelagic]], and the general larval form is kept throughout life. Also the class [[Thaliacea]] is pelagic throughout their lives and may have complex lifecycles. In this class a free living larval stage is absent: Doliolids and pyrosomatids are viviparous–lecithotrophic, and salpids are viviparous–matrotrophic. Only some species of doliolids still have a rudimentary tailed tadpole stage, which is never free-living and lacks a brain.<ref>{{cite journal |pmc=5098176 |year=2015 |last1=Ostrovsky |first1=A. N. |last2=Lidgard |first2=S. |last3=Gordon |first3=D. P. |last4=Schwaha |first4=T. |last5=Genikhovich |first5=G. |last6=Ereskovsky |first6=A. V. |title=Matrotrophy and placentation in invertebrates: A new paradigm |journal=Biological Reviews of the Cambridge Philosophical Society |volume=91 |issue=3 |pages=673–711 |doi=10.1111/brv.12189 |pmid=25925633 }}</ref><ref name=Dorit>{{cite book |last1=Dorit |first1=R.L. |last2=Walker |first2=W.F. |last3=Barnes |first3=R.D. |name-list-style=amp |year=1991 |title=Zoology |publisher=Saunders College Publishing |isbn=978-0-03-030504-7 |pages=[https://archive.org/details/zoology0000dori/page/802 802–804] |url=https://archive.org/details/zoology0000dori |url-access=registration |via=archive.org}}</ref><ref>{{Cite book |last=Schlosser |first=Gerhard |url=https://books.google.com/books?id=rPMxEAAAQBAJ&dq=Doliolids+never+free+and+lacks+a+brain&pg=PP30 |title=Evolutionary Origin of Sensory and Neurosecretory Cell Types: Vertebrate Cranial Placodes, volume 2 |date=2021-06-17 |publisher=CRC Press |isbn=978-1-000-36913-7 |language=en}}</ref>

Tunicates have a well-developed [[heart]] and [[circulatory system]]. The heart is a double U-shaped tube situated just below the gut. The blood vessels are simple connective tissue tubes, and their blood has several types of [[Blood cell|corpuscle]]. The blood may appear pale green, but this is not due to any respiratory pigments, and oxygen is transported dissolved in the [[Blood plasma|plasma]]. Exact details of the circulatory system are unclear, but the gut, pharynx, gills, gonads, and nervous system seem to be arranged in series rather than in parallel, as happens in most other animals. Every few minutes, the heart stops beating and then restarts, pumping fluid in the reverse direction.<ref name=Ruppert/>

Tunicate [[blood]] has some unusual features. In some species of [[Ascidiidae]] and [[Perophoridae]], it contains high concentrations of the transitional metal [[vanadium]] and [[Vanabins|vanadium-associated proteins]] in [[vacuole]]s in blood cells known as [[vanadocyte]]s. Some tunicates can concentrate vanadium up to a level ten million times that of the surrounding seawater. It is stored in a +3 oxidation form that requires a [[pH]] of less than 2 for stability, and this is achieved by the vacuoles also containing [[sulfuric acid]]. The vanadocytes are later deposited just below the outer surface of the tunic, where their presence is thought to deter [[predation]], although it is unclear whether this is due to the presence of the metal or low pH.<ref>{{cite journal |first1=S. |last1=Odate |first2=J.R. |last2=Pawlik |name-list-style=amp |year=2007 |title=The role of vanadium in the chemical defense of the solitary tunicate, ''Phallusia nigra'' |journal=Journal of Chemical Ecology |volume=33 |issue=3 |pages=643–654 |doi=10.1007/s10886-007-9251-z |pmid=17265174 |bibcode=2007JCEco..33..643O |s2cid=116921}}</ref> Other species of tunicates concentrate [[lithium]], [[iron]], [[niobium]], and [[tantalum]], which may serve a similar function.<ref name=Ruppert>{{cite book |last1=Ruppert |first1=E.E. |last2=Fox |first2=R.S. |last3=Barnes |first3=R.D. |name-list-style=amp |year=2004 |title=Invertebrate Zoology |edition=7th |publisher=Cengage Learning |isbn=978-81-315-0104-7 |pages=940–956 }}</ref> Other tunicate species produce distasteful [[organic compound]]s as [[chemical defense]]s against predators.<ref>{{cite journal |first1=D.P. |last1=Pisut |first2=J.R. |last2 = Pawlik |name-list-style=amp |year=2002 |title=Anti-predatory chemical defenses of ascidians: Secondary metabolites or inorganic acids? |journal=Journal of Experimental Marine Biology and Ecology |volume=270 |issue=2 |pages=203–214 |doi=10.1016/S0022-0981(02)00023-0 |bibcode=2002JEMBE.270..203P |citeseerx=10.1.1.558.3639}}</ref>

Tunicates lack the kidney-like [[Nephridium|metanephridial]] organs typical of [[deuterostome]]s. Most have no excretory structures, but rely on the diffusion of [[ammonia]] across their tissues to rid themselves of nitrogenous waste, though some have a simple excretory system. The typical [[Kidney|renal]] organ is a mass of large clear-walled [[Vesicle (biology and chemistry)|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 [[Uric acid|urate crystals]], and do not have any obvious means of disposing of the material during their lifetimes.<ref name=Dorit/>

Adult tunicates have a hollow cerebral ganglion, equivalent to a brain, and a hollow structure known as a neural gland. Both originate from the embryonic neural tube and are located between the two siphons. Nerves arise from the two ends of the ganglion; those from the anterior end innervate the buccal siphon and those from the posterior end supply the rest of the body, the atrial siphon, organs, gut and the musculature of the body wall. There are no sense organs but there are sensory cells on the siphons, the buccal tentacles and in the atrium.<ref name=Ruppert/>

Tunicates are unusual among animals in that they produce a large fraction of their tunic and some other structures in the form of [[cellulose]]. The production in animals of cellulose is so unusual that at first some researchers denied its presence outside of plants, but the tunicates were later found to possess a functional cellulose [[synthase|synthesizing enzyme]], encoded by a gene horizontally transferred from a bacterium.<ref>{{cite journal |author1=Matthysse, A.G. |author2=Deschet, K. |author3=Williams, M. |author4=Marry, M. |author5=White, A.R. |author6=Smith, W.C. |name-list-style=amp |year=2004 |title=A functional cellulose synthase from ascidian epidermis |journal=[[Proceedings of the National Academy of Sciences]] |volume=101 |issue=4 |pages=986–991 |doi=10.1073/pnas.0303623101 |doi-access=free |pmid=14722352 |pmc=327129 |bibcode=2004PNAS..101..986M}}</ref> When, in 1845, [[Carl Schmidt (chemist)|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 is now recognized as cellulose rather than any alternative substance.<ref>{{cite journal |author1=Hirose, E. |author2=Nakashima, K. |author3=Nishino, A. |name-list-style=amp |year=2011 |title=Is there intracellular cellulose in the appendicularian tail epidermis? A tale of the adult tail of an invertebrate chordate |journal=Communicative & Integrative Biology |volume=4 |issue=6 |pages=768–771 |doi=10.4161/cib.17757|pmid=22446551 |pmc=3306355 }}</ref><ref>{{cite journal |author1=Sasakura, Y. |author2=Ogura, Y. |author3=Treen, N. |display-authors=etal |year=2016 |title=Transcriptional regulation of a horizontally transferred gene from bacterium to chordate |journal=[[Proceedings of the Royal Society B]] |volume=283 |issue=1845 |page=20161712 |doi=10.1098/rspb.2016.1712 |pmid=28003446 |pmc=5204163 }}</ref><ref>{{cite journal |author1=Sasakura, Y. |author2=Nakashima, K. |author3=Awazu, S. |author4=Matsuoka, T. |author5=Nakayama, A. |author6=Azuma, J. |author7=Satoh, N. |name-list-style=amp |year=2005 |title=Transposon-mediated insertional mutagenesis revealed the functions of animal cellulose synthase in the ascidian ''Ciona intestinalis'' |journal=[[Proceedings of the National Academy of Sciences]] |volume=102 |issue=42 |pages=15134–15139 |doi=10.1073/pnas.0503640102 |pmid=16214891 |pmc=1257696 |bibcode=2005PNAS..10215134S |doi-access=free }}</ref>

<gallery mode="packed" heights="160px" style="float:center;">
File:Oikopleura (Vexillaria) cophocerca 001.png|''[[Oikopleura cophocerca]]'' in its "house". Arrows indicate water movement and (x) the lateral reticulated parts of the house.
File:Ascidians.jpg|Blue sea squirts from the genus ''[[Rhopalaea]]''.
File:Ascidian (Rhopalaea Crassa) (4 cm).png| Fluorescent-colored sea squirts, ''[[Rhopalaea crassa]]''.
File:Sea Squirts Didemnum molle.jpg| ''[[Didemnum molle]]''.
</gallery>