Neomenia yamamotoi, a large (11 cm) solenogaster from the
Northern Pacific. Picture: Roger Clark, Jacksonville Shell Club.
As are their Caudofoveata relatives, the solenogasters also are among the little known shell-less molluscs ("Aplacophora"). But other than for example slugs, shell-less gastropods, during whose evolution the shell has secondarily disappeared, the solenogasters with about 230 species today still belong in the vicinity of those primordial molluscs which had not yet developed a shell. Instead, solenogasters have a sturdy outer skin, the cuticula, which is reinforced with calcareous spikes, the so-called spiculi. To the naked eye, those spikes give a solenogaster a glossy, furry appearance.
Different from the ground-living caudofoveatans, solenogasters in general live on cnidarians, for example corals, whose polyps they eat. Most species grab their prey using their radula and swallow it. Others puncture their skin and suck the cell fluid from them. In those species the radula then is often reduced, which is the case for about one third of all solenogaster species. So the radula is present also among the Solenogastres, pointing once again towards the conclusion that the radula is a very old mollusc character.
A very special character of solenogasters is a ventral furrow with a short, rudimentary foot, which is completely reduced among their relatives, the caudofoveatans.
Glands in the ventral furrow of a solenogaster also produce mucus, so the animal can crawl like a snail does. A head shield, like among the caudofoveatans, is absent, but solenogasters instead have a head groove with numerous sense cells. The group's scientific name, Solenogastres, literally means "animals with a furrowed belly", also a hint towards the characteristic ventral furrow of those creatures.
An unidentified solenogaster (Neomenia spec.). The furry
appearance is due to a large number of tiny calcareous spicules.
Many solenogasters (Epimenia babai) live on corals
they also feed on. Source: Atwiki.jp.
As do the caudofoveatans, the solenogasters also have a small pallial caity at their body's end, in which the double gills are situated. In the solenogasters, too, the dorsal gonad's efferent duct mouths into the pericardium and from there into the pallial cavity.
In contrary to the caudofoveatans, solenogasters are hermaphrodites, which change their sex during their life (similar to slipper limpets): Younger specimens live as males, while the older ones are females. Fertilisation takes place internally after a copulation, unlike the external fertilisation among caudofoveatans.
However, both, have in common the larval development passing a planktontic larval stage of the trochophora type. In this regard, caudofoveatans and solenogasters still are very near the segmented worms (Annelida), which also develop passing a similar larval stage.
Salvini-Plawen, L.; Mizzaro-Wimmer, M.: "Praktische Malakologie - Beiträge zur vergleichend anatomischen Bearbeitung der Mollusken", Vienna 2001.