aplacophora

Caudofoveates generally burrow into the substrate while solenogasters are usually epibenthic. Both taxa are most common in water regions deeper than {{convert|20|m|ft}} where some species may reach densities up to four or five specimens per square metre (three or four per square yard). Solenogasters are typically carnivores feeding on cnidarians or sometimes annelids or other taxa while caudofoveates are mostly detritovores or feed on foraminiferans.{{Cite journal | doi = 10.4003/006.031.0110| title = Aplacophoran Mollusks—Still Obscure and Difficult?| journal = American Malacological Bulletin| volume = 31| pages = 181–187| year = 2013| last1 = Todt | first1 = C. | s2cid = 85911879}}

File:Epimenia verrucosa.jpg

Aplacophorans are worm-like animals, with little resemblance to most other molluscs. They have no shell, although small calcified spicules are embedded in the skin; these spicules are occasionally coated with an organic pellicle that is presumably secreted by microvilli.{{cite journal | last = Haas | first = Winfried | pages = 403–418 | date = 1981 | journal = Malacologia | title = Evolution of calcareous hardparts in primitive molluscs | volume = 21 | url = https://archive.org/stream/malacologia211981inst/malacologia211981inst_djvu.txt }} Caudofoveates lack a foot while solenogasters have a narrow foot which lacks intrinsic musculature. The mantle cavity is reduced into a simple cloaca, into which the anus and excretory organs empty, and is located at the posterior of the animal. The head is rudimentary, and has no eyes or tentacles. The cuticle of both subclasses is chitinous,{{Cite journal | last1 = Peters | first1 = W. | title = Occurrence of chitin in mollusca | doi = 10.1016/0305-0491(72)90117-4 | journal = Comparative Biochemistry and Physiology B | volume = 41 | issue = 3 | pages = 541–550| year = 1972 }} and has an irregular texture.{{cite journal | year = 2012 | title = Formation and morphology of epidermal sclerites from a deep-sea hydrothermal vent solenogaster (Helicoradomenia sp., Solenogastres, Mollusca) | journal = Zoomorphology | volume = 132| pages = 1–9| doi = 10.1007/s00435-012-0168-x | last1 = Kingsley | first1 = Roni J. | last2 = Froelich | first2 = Jenna | last3 = Marks | first3 = Carolyn B. | last4 = Spicer | first4 = Lori M. | last5 = Todt | first5 = Christiane | s2cid = 15935925 }} Spines bear an organic matrix.{{Cite journal | last1 = Kingsley | first1 = R. J. | last2 = Marks | first2 = C. B. | doi = 10.1017/S1431927608088405 | title = Spicule Morphology and Formation in Helicoradomenia acredema (Mollusca: Aplacophora) | journal = Microscopy and Microanalysis | volume = 14 | issue = S2 | year = 2008 | pages=1450–1451| bibcode = 2008MiMic..14S1450K | s2cid = 135492113 }} Sclerites can be hollow or solid; the void within the sclerites of some species fills during growth. Sclerites generally form within the cuticle, protruding through when they are fully grown. They probably start life as amorphous calcium carbonate, which the organic matrix coaxes into an aragonitic habit as the spines mature. Sclerites can be shaped as simple spines, straight, curved, keeled, striated or hooked; or as cupped blades; more complex arrangements are common in copulatory spicules.{{Cite journal|last=Scheltema|first=Amelie H.|date=1985|title=The Aplacophoran Family Prochaetodermatidae in the North American Basin, Including Chevroderma n.g. and Spathoderma n.g. (Mollusca; Chaetodermomorpha)|journal=The Biological Bulletin|volume=169|issue=2|pages=484–529|doi=10.2307/1541497|jstor=1541497 }}{{cite journal | pmid=22815372 | year=2012 | last1=Scheltema | first1=A. H. | last2=Schander | first2=C | last3=Kocot | first3=KM | title=Hard and soft anatomy in two genera of Dondersiidae (Mollusca, Aplacophora, Solenogastres) | volume=222 | issue=3 | pages=233–69 | journal=The Biological Bulletin | doi=10.1086/bblv222n3p233| hdl=1912/5273 | s2cid=28029055 | hdl-access=free }} In several species of solenogaster, sclerites change morphology during growth; young specimens might bear flat, solid, scale-like sclerites, to be replaced with longer hollow spine-like sclerites in adults.{{Cite journal | last1 = Todt | first1 = C. | last2 = Wanninger | first2 = A. | doi = 10.1186/1742-9994-7-6 | title = Of tests, trochs, shells, and spicules: Development of the basal mollusk Wirenia argentea (Solenogastres) and its bearing on the evolution of trochozoan larval key features | journal = Frontiers in Zoology | volume = 7 | issue = 1 | pages = 6 | year = 2010 | pmid = 20181015| pmc =2828982 | doi-access = free }}

The relationship with other molluscs, however, is apparent from some features of the digestive system; aplacophorans possess both a radula and a style. A variety of radular forms and functions exist.{{Cite journal | last1 = Todt | first1 = C. | last2 = Von Salvini-Plawen | first2 = L. | doi = 10.1111/j.1744-7410.2005.00023.x | title = The digestive tract of Helicoradomenia (Solenogastres, Mollusca), aplacophoran molluscs from the hydrothermal vents of the East Pacific Rise | journal = Invertebrate Biology | volume = 124 | issue = 3 | pages = 230–253 | year = 2005 }} Solenogasters are hermaphroditic and assumed to have internal fertilization, in contrast to caudofoveates which have two sexes, and reproduce by external fertilization. During development, the mantle cavity of the larva curls up and closes, creating the worm-like form of the adult.{{cite book |author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia, PA|page= 389|isbn= 978-0-03-056747-6}} Caudofoveates also differ from Solenogasters in having a head shield and a body that is differentiated into three sections.[https://www.routledgehandbooks.com/doi/10.1201/9781351115254-3 Biology and Evolution of the Mollusca]

This class was once classified as sea cucumbers in the echinoderms. In 1987, they were officially recognized as molluscs and given their own class. It consists of two clades: the Solenogastres and the Caudofoveata. It has been considered polyphyletic{{Cite journal| last3=Lindgren, A. R.| last1=Giribet | first3=A. R. | first1=G.| last4=Huff, S. W. | first4=S. W. | first2=A.| last5 = Schrödl, M | first5=M.| last6 = Nishiguchi, M. K. | first6=M. K.| title = Evidence for a clade composed of molluscs with serially repeated structures: monoplacophorans are related to chitons| last2= Okusu, A| journal = Proceedings of the National Academy of Sciences of the United States of America| volume = 103| issue = 20| pages = 7723–7728| date=May 2006 | pmid = 16675549| pmc = 1472512| doi = 10.1073/pnas.0602578103|bibcode = 2006PNAS..103.7723G | doi-access=free }} but more recent molecular evidence supports it as a monophyletic clade.{{Cite journal | last1 = Kocot | first1 = K. M. | last2 = Cannon | first2 = J. T. | last3 = Todt | first3 = C. | last4 = Citarella | first4 = M. R. | last5 = Kohn | first5 = A. B. | last6 = Meyer | first6 = A. | last7 = Santos | first7 = S. R. | last8 = Schander | first8 = C. | last9 = Moroz | first9 = L. L. | last10 = Lieb | first10 = Bernhard | last11 = Halanych | first11 = Kenneth M. |display-authors=9| doi = 10.1038/nature10382 | title = Phylogenomics reveals deep molluscan relationships | journal = Nature | volume = 477 | issue = 7365 | pages = 452–456 | year = 2011 | pmid = 21892190| pmc = 4024475|bibcode = 2011Natur.477..452K }}{{Cite journal | last1 = Smith | first1 = S. A. | last2 = Wilson | first2 = N. G. | last3 = Goetz | first3 = F. E. | last4 = Feehery | first4 = C. | last5 = Andrade | first5 = S. N. C. S. | last6 = Rouse | first6 = G. W. | last7 = Giribet | first7 = G. | last8 = Dunn | first8 = C. W. | doi = 10.1038/nature10526 | title = Resolving the evolutionary relationships of molluscs with phylogenomic tools | journal = Nature | volume = 480 | issue = 7377 | pages = 364–367 | year = 2011 | pmid = 22031330|bibcode = 2011Natur.480..364S | s2cid = 4321076 }}

The affinities of Aplacophorans have long been uncertain. Molecular and fossil evidence seemed to put Aplacophorans in the clade Aculifera, as a sister group to Polyplacophora.{{Cite journal| last1 = Sigwart | first1 = J. D.| last2 = Sutton | first2= M. D.| title = Deep molluscan phylogeny: synthesis of palaeontological and neontological data| journal = Proceedings of the Royal Society B: Biological Sciences| issue = 1624| volume = 274| pages = 2413–2419| date =Oct 2007| pmid = 17652065| pmc = 2274978| doi = 10.1098/rspb.2007.0701}} For a summary, see {{cite web| title=The Mollusca | publisher=University of California Museum of Paleontology| url=http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php| access-date=2 October 2008}}{{cite journal |jstor=1542380 |last=Scheltema|first=A.H. |title=Aplacophora as Progenetic Aculiferans and the Coelomate Origin of Mollusks as the Sister Taxon of Sipuncula |journal=The Biological Bulletin|date=1 February 1993|volume=184|issue=1|pages=57–78 |doi=10.2307/1542380 |pmid=29300613|url=https://www.biodiversitylibrary.org/part/37201}}{{Cite journal | last1 = Vinther | first1 = J. | last2 = Sperling | first2 = E. A. | last3 = Briggs | first3 = D. E. G. | author-link3 = Derek Briggs | last4 = Peterson | first4 = K. J. | title = A molecular palaeobiological hypothesis for the origin of aplacophoran molluscs and their derivation from chiton-like ancestors | journal = Proceedings of the Royal Society B: Biological Sciences | year = 2011 | doi = 10.1098/rspb.2011.1773 | volume=279 | issue = 1732 | pages=1259–1268 | pmid=21976685 | pmc=3282371}} The discovery of Kulindroplax in 2012, a fossil aplacophoran with a polyplacophoran-like armour, strongly supports this hypothesis, and shows that aplacophorans evolved from progenitors that bore valves.{{Cite journal | last1 = Sutton | first1 = M. D. | last2 = Briggs | first2 = D. E. G. | last3 = Siveter | first3 = D. J. | last4 = Siveter | first4 = D. J. | last5 = Sigwart | first5 = J. D. | title = A Silurian armoured aplacophoran and implications for molluscan phylogeny | doi = 10.1038/nature11328 | journal = Nature | volume = 490 | issue = 7418 | pages = 94–97 | year = 2012 | pmid = 23038472|bibcode = 2012Natur.490...94S | s2cid = 4422386 }}

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{{Wikispecies|Aplacophora}}

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Category:Aplacophorans

Category:Mollusc classes