Some recent literature and recent molecular evidence,[when?] indicates that the Aplacophora may bepolyphyletic, and therefore some taxonomists may divide Solenogastres and Caudofoveata into separate classes.[2]
In contrast to many other mollusc classes, aplacophorans have no shell, and are instead covered byaragoniticsclerites (calcareousspicules), which can be solid or hollow. These spicules can be arranged perpendicular to one another within the cuticle to form a skeleton, stick up to form a palisade, or can lie flat against the cuticle.[3]
80% of solenogasterspecies have aradula, while in others it is secondarily lost. The radula may bear one or more teeth per row; where there is more than one tooth, there is no central radular tooth.[3] The radula grows by dividing existing teeth in two, or by adding a new tooth at the center of the radular row.[3] Thesalivary glands are very elaborate, and are an important characteristic for taxonomy. Next to the mouth they have a unique sense organ, the vestibulum.
Solenogastres do not have true ctenidia, though their gill-like structures may resemble them.[4]
During development, many Solenogastres are covered by a spinyscleritome comprising spines or scale-like plates. This has been likened to thehalwaxiid scleritome.[5]
Sclerites ofEpimenia start out solid before developing a hollow stem that subsequently solidifies.[6]
Solenogastres can be found in a diverse range of habitats across the world, from the coast to the deep ocean.[7]
Solenogastres feed oncnidaria andctenophores, either sucking their bodily fluids or eating their tissue.[8] They do not use their radulae to rasp prey, as some other molluscs do.[9]
There is some uncertainty regarding the phylogenetic position of the Solenogastres. Traditionally considered to be the most basal molluscan group and the sister group to the Caudofoveata, alternatives to both of these statements have been proposed.[5] Some molecular datasets plot Solenogastres as an outgroup to Mollusca.[10] However, there are cryptic species which are hard to delineate due to there being a lack of multiple morphological characteristics.[11]
^e.g.Todt, C.; Okusu, A.; Schander, C. & Schwabe, E. (2008). "Solenogastres, Caudofoveata and Polyplacophora.". In Ponder, W. & Lindberg D. (eds.).Phylogeny and Evolution of the Mollusca. University of California Press.ISBN978-0-520-25092-5.
^Wilbur, Karl M.; Trueman, E.R.; Clarke, M.R., eds. (1985), "2. Early evolution and the Primitive Groups",The Mollusca, vol. 10. Evolution, New York: Academic Press,ISBN0-12-728702-7
^Scheltema, A. H.; Jebb, M. (1994). "Natural history of a solenogaster mollusc from Papua New Guinea,Epimenia australis(Thiele) (Aplacophora: Neomeniomorpha)".Journal of Natural History.28 (6): 1297.Bibcode:1994JNatH..28.1297S.doi:10.1080/00222939400770661.
^Wilson, N.; Rouse, G.; Giribet, G. (2010). "Assessing the molluscan hypothesis Serialia (Monoplacophora+Polyplacophora) using novel molecular data".Molecular Phylogenetics & Evolution.54 (1):187–193.doi:10.1016/j.ympev.2009.07.028.PMID19647088.
