The discovery of marine representatives of the Valvatoidea (Ectobranchia) by Ponder in 1990 established this clade as basal Heterobranchia, formerly known only from freshwater habitats. We investigated the anatomy of two marine microgastropods originally classified as Skeneidae by shell and radula characters and later transferred to the Valvatoidea as family Hyalogyrinidae: Hyalogyrina depressa from deep waters off Japan and Xenoskenea pellucida from Mediterranean shallow water. We applied computer aided 3D-reconstructions (software: AMIRA™) based on semithin section series.
Both species have a tapered snout with a pair of tentacles, an anteriorly bifurcate foot, and a metapodium with a large mass of calcium cells. The mantle cavity contains an osphradium, a bipectinate, ciliated gill without bursicles or skeletal rods, and a left, pallial kidney. The rectum bypasses the monotocardian heart and shows several loops in the pallial roof. A complex, hermaphroditic genital system suggests internal fertilisation. The rhipidoglossate radula apparatus lacks cartilages, the salivary glands are long and tubular, the stomach shows a gastric shield. The nervous system is epiathroid and streptoneurous, each statocyst contains a single statolith. The eye-less Hyalogyrina depressa has a ciliated pallial tentacle at the right side. Xenoskenea pellucida has a small copulatory organ behind the right cephalic tentacle, a metapodial tentacle, and simple lens eyes.
These anatomical data and the hyperstrophic larval shells all reflect basal heterobranch conditions. The metapodial calcium cells and the looped pallial rectum are interpreted as synapomorphies for the Hyalogyrinidae. Their rhipidoglossate radula and the lack of cartilages suggest that the Heterobranchia as a whole emerged from the rhipidoglossate rather than from the taenioglossate level of gastropod evolution. Accordingly, a change of function in the supporting apparatus (replacement of cartilages by a muscular mass) predates the change of the radular type itself at the beginning of heterobranch evolution.