Belongs within: Aculifera.
Contains: Simrothiellidae.
The Solenogastres are a group of mostly small molluscs characterised by a vermiform body shape, narrow ventral foot, lack of a shell, and a covering of aragonitic sclerites (Yap-Chiongco et al. 2024).
Scleritic ‘worms’
Published 14 April 2026
Mention a ‘mollusc’ to many people and they will immediately picture a snail or some other gastropod. A smaller, but still substantial, contingent will be more of a bivalve bent, picturing a mussel or clam. A few smug weirdos may think of an octopus or squid. But there will be very few out there who immediately call to mind a solenogaster.
Solenogastres are a small class of marine molluscs with only about 300 species described to date (though, as always, many more probably remain unrecognised). The majority of species are small, the smallest being less than a millimetre in length, but the largest species may exceed a foot (Todt et al. 2008). All species are worm-like in appearance, lacking a cohesive shell but instead having the body covered by a chitinous cuticle bearing a carpet of aragonitic sclerites. The foot is reduced to a narrow sole in a ventral groove. The digestive system is fairly simple compared to other molluscs, usually (but not always) with a radula in the foregut. Most solenogastres are predators, commonly on colonial cnidarians, but some feed on bacteria. The relatively weak radula is used for grasping and gathering rather than directly rasping food; in the large-bodied genus Epimenia, the radula grasps onto coral polyps that are then sucked in by the pharynx (Fontoura-da-Silva et al. 2013).
Because of their simple structure and lack of a shell, Solenogastres have long been presumed to occupy a fairly basal position in mollusc phylogeny. Earlier authors united them with another group of shell-less, spiculate molluscs, the Caudofoveata, into a class Aplacophora, but these came to be treated as separate classes on the ground that the similarities between the two were probably shared primitive characters only. Nevertheless, many recent studies have supported Aplacophora as monophyletic after all, and likely forming a broader clade with the Polyplacophora (chitons). The Solenogastres have been divided between four orders on the basis of features such as cuticle, spicule and foregut structure, but phylogenetic studies suggest that many of these features are polyphyletic (Yap-Chingco et al. 2024). Identification of solenogaster species is commonly a challenge, requiring close histological examination. This, even more than their small size, is probably a major factor underlying their obscurity.
Systematics of Solenogastres
<==Solenogastres [Aplotegmentaria, Neomeniida, Neomeniina, Neomeniomorpha, Pachytegmentaria, Ventroplicida]
| i. s.: Teuglaherpia tasmanicaVN05
| Spengelomenia bathybiaBRW98
| OcheyoherpiaBRW98
|--Alexandromenia [Amphimeniidae]Y-CB24
| `--A. crassaY-CB24
`--+--MeiomeniidaeY-CB24
| |--Meiomenia Morse 1979S-P86
| | |--M. arenicola Salvini-Plawen & Sterrer 1985S-P86
| | `--M. swedmarki Morse 1979S-P86
| `--Meioherpia Salvini-Plawen 1985S-P86
| |--M. atlantica Salvini-Plawen, Rieger & Sterrer 1985S-P86
| `--M. stygalis Salvini-Plawen & Sterrer 1985S-P86
`--+--PholidoskepiaY-CB24
| |--Gymnomeniidae [Wireniidae]Y-CB24
| | |--Wirenia argentea Odner 1921KC11, Y-CB24
| | `--Gymnomenia pellucidaY-CB24
| `--DondersiidaeY-CB24
| | i. s.: NematomeniaBRW98
| | |--N. banyulensisA99
| | |--N. flavensBRW98
| | `--N. platypodaBRW98
| |--+--Macellomenia [Macellomeniidae]Y-CB24
| | | |--M. adenota Salvini-Plawen 2003F04
| | | `--M. schanderi Kocot & Todt 2014Y-CB24
| | `--Dondersia todtaeY-CB24
| `--+--Stylomenia sulcodoryataY-CB24
| `--+--Micromenia fodiens Schwabl 1955Y-CB24
| `--Helluoherpia aegiriY-CB24
`--+--+--Phyllomenia [Phyllomeniidae, Sterrofustia]Y-CB24
| `--+--PruvotinidaeY-CB24
| | |--EleutheromeniaBRW98
| | |--Hypomenia sanjuanensis Kocot & Todt 2014Y-CB24
| | `--Scheltemaia mimus Scheltema & Schander 2000Y-CB24
| `--LepidomeniidaeY-CB24
| |--Tegulaherpia tasmanicaBRW98
| `--Lepidomenia Kowalevsky 1883S-P86
| `--L. hystrix Marion & Kowalevsky 1886S-P86
`--+--Neomenia [Neomeniadae, Neomeniamorpha]Y-CB24
| |--N. carinata Tullberg 1875Y-CB24
| |--N. herwigi Kaiser 1976KC11
| |--N. megatrapezata Salvini-Plawen & Paar-Gausch 2004Y-CB24
| `--N. permagna Salvini-Plawen 1978KC11
`--+--Apodomenia [Apodomeniidae]Y-CB24
| `--A. enigmatica Kocot, Todt et al. 2019Y-CB24
`--CavibeloniaY-CB24
|--SimrothiellidaeY-CB24
|--AcanthomeniidaeS-P04
|--UrgorriaF04 [RhopalomeniidaeS-P04]
| `--U. monoplicata Salvini-Plawen 2003F04
|--StrophomeniidaeS-P04
|--Notomenia Thiele 1897 [Notomeniidae]S-P04
| `--*N. clavigera Thiele 1897S-P04
|--Syngenoherpia [Syngenoherpiidae]S-P04
| `--S. intermediaS-P04
|--Unciherpiidae [Pararrhopaliidae]S-P04
| |--UncimeniaS-P04
| |--SialoherpiaS-P04
| `--PararrhopaliaBRW98
|--RhipidoherpiidaeS-P04
| |--RhipidoherpiaS-P04
| `--Thieleherpia von Salvini-Plawen 2004S-P04
| `--*T. thulensis (Thiele 1900) [=Proneomenia thulensis]S-P04
|--Drepanomenia Heath 1911 [Drepanomeniidae]S-P04
| |--*D. vampyrella (Heath 1905) [=Neomenia vampyrella]S-P04
| |--D. incrustata (Koren & Danlielssen 1877)S-P04
| |--D. perticata Salvini-Plawen 1978S-P04
| |--D. pontisquamata von Salvini-Plawen 2004S-P04
| `--D. tenuitecta von Salvini-Plawen 2004S-P04
`--ProneomeniidaeS-P04
|--Proneomenia Hubrecht 1880Y-CB24, S-P04
| |--P. australis Thiele 1897H09
| |--P. custodiensY-CB24
| `--P. sluiteriY-CB24
`--+--Epimenia Nierstrasz 1908Y-CB24 [EpimeniidaeS-P04]
| |--E. arabicaBRW98
| |--E. australisS96
| |--E. babai Salvini-Plawen 1997Y-CB24
| |--E. oshimaiBRW98
| `--E. verrucosaA99
`--Dorymenia Heath 1911Y-CB24, S-P04
|--*D. acuta Heath 1911S-P04
|--D. antarcticaA99
|--D. harpagataS-P04
|--D. quincarinata (Ponder 1970) [=Proneomenia quincarinata]S-P04
|--D. sarsiiBRW98
`--D. tricarinataS-P04
Nomen nudum: Lepidomenia swedmarki Salvini-Plawen 1986 [=L. hystrix Swedmark 1956 non Marion & Kowalevsky 1886]S-P86
*Type species of generic name indicated
References
[A99] Ax, P. 1999. Das System der Metazoa II. Ein Lehrbuch der phylogenetischen Systematik. Gustav Fisher Verlag: Stuttgart (translated: 2000. Multicellular Animals: The phylogenetic system of the Metazoa vol. 2. Springer).
[BRW98] Beesley, P. L., G. J. B. Ross & A. Wells (eds) 1998. Fauna of Australia vol. 5. Mollusca: The Southern Synthesis. Part A. Australian Biological Resources Study: Canberra.
[F04] Fernández, J. 2004. Noticia de nuevos táxones para la ciencia en el ámbito Íbero-Balear y Macaronésico. Nuevos táxones animales descritos en la península Ibérica y Macaronesia desde 1994 (VIII). Graellsia 60 (2): 217–243.
Fontoura-da-Silva, V., R. J. de Souza Dantas & C. H. Soares Caetano. 2013. Foraging tactics in Mollusca: a review of the feeding behavior of their most obscure classes (Aplacophora, Polyplacophora, Monoplacophora, Scaphopoda and Cephalopoda). Oecologia Australis 17 (3): 358–373.
[H09] Hedley, C. 1909. The Marine Fauna of Queensland: Address by the President of Section D. Australasian Association for the Advancement of Science: Brisbane.
[KC11] Kocot, K. M., J. T. Cannon, C. Todt, M. R. Citarella, A. B. Kohn, A. Meyer, S. R. Santos, C. Schander, L. L. Moroz, B. Lieb & K. M. Halanych. 2011. Phylogenomics reveals deep molluscan relationships. Nature 477: 452–456.
[S-P86] Salvini-Plawen, L. v. 1986. Lower Mollusca. In: Botosaneanu, L. (ed.) Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) pp. 148–152. E. J. Brill/Dr W. Backhuys: Leiden.
[S-P04] Salvini-Plawen, L. von. 2004. Contributions to the morphological diversity and classification of the order Cavibelonia (Mollusca: Solenogastres). Journal of Molluscan Studies 70: 73–93.
[S96] Scheltema, A. H. 1996. Phylogenetic position of Sipuncula, Mollusca and the progenetic Aplacophora. In: Taylor, J. D. (ed.) Origin and Evolutionary Radiation of the Mollusca pp. 53–58. Oxford University Press: Oxford.
Todt, C., A. Okusu, C. Schander & E. Schwabe. 2008. Solenogastres, Caudofoveata, and Polyplacophora. In: Ponder, W. F., & D. R. Lindberg (eds) Phylogeny and Evolution of the Mollusca pp. 71–96. University of California Press.
[VN05] Vinther, J., & C. Nielsen. 2005. The Early Cambrian Halkieria is a mollusc. Zoologica Scripta 34: 81–89.
[Y-CB24] Yap-Chiongco, M. K., F. S. Bergmeier, N. G. Roberts, K. M. Jörger & K. M. Kocot. 2024. Phylogenomic reconstruction of Solenogastres (Mollusca, Aplacophora) informs hypotheses on body size evolution. Molecular Phylogenetics and Evolution 194: 108029.