The Solenogastres common name

Common name

A common name of a taxon or organism is a name in general use within a community; it is often contrasted with the scientific name for the same organism...

 solenogasters, are a subclass of worm-like, small, shell-less molluscs (Aplacophora), the other subclass being the Caudofoveata

Caudofoveata

Caudofoveata is a small class within the phylum Mollusca, also known as Chaetodermomorpha. The class is often combined with Solenogastres and termed Aplacophora. However, such a grouping is not monophyletic; molecular data suggests that the Caudofoveata are a sister group to the cephalopods.-...

 (Chaetodermomorpha).

Morphology

In contrast to all other molluscan classes, the Aplacophora have no shell, and are instead covered by aragonitic

Aragonite

Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...

 sclerites (calcareous

Calcareous

Calcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.-In zoology:...

 spicule

Spicule

Spicules are tiny spike-like structures of diverse origin and function found in many organisms, such as the copulatory spicules of certain nematodes or the grains on the skin of some frogs.In sponges, spicules perform a structural function....

s), which can be solid or hollow. These spicules can be arranged perpendicular one another within the cuticle to form a skeleton; or can stick up to form a pallisade; or can lay flat against the cuticle.

80% of solenogaster species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 have a radula

Radula

The radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...

, 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. The radula grows by dividing existing teeth in two, or by adding a new tooth at the centre of the radular row. The salivary gland

Salivary gland

The salivary glands in mammals are exocrine glands, glands with ducts, that produce saliva. They also secrete amylase, an enzyme that breaks down starch into maltose...

s are very elaborate, and are an important character for taxonomy. Next to the mouth they have a unique sense organ, the vestibulum.

The solenogastres do not have true ctenidia, although their gill-like structures resemble them.

Development

During development many solenogastres are covered by a spiny scleritome comprising spines or scale-like plates; this has been likened to the halwaxiid scleritome.

Sclerites of Epimenia

Epimenia

Epimenia is a genus of cavibelonian solenogaster. During development, its sclerites start out as solid tips, then a hollow stalk develops that is subsequently infilled....

start out solid before developing a hollow stem that subsequently solidifies.

Diet

Solenogasters feed on cnidaria and ctenophores, either sucking their bodily fluids or eating their tissue. They do not use their radulae to rasp prey, as other molluscs do.

Phylogeny

There is some uncertainty regarding the phylogenetic position of the solenogasters. 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 on various lines of evidence. Indeed, some molecular datasets plot Solenogastres as an outgroup to Mollusca.

Further reading

• Meyer A., Todt C., Mikkelsen N. T. & Lieb B. (2010). "Fast evolving 18S rRNA sequences from Solenogastres (Mollusca) resist standard PCR amplification and give new insights into mollusk substitution rate heterogeneity". BMC Evolutionary Biology 10: 70.