Systematic of Opistobranchia

We find different species of what we call "Seaslug" in five different order of Opistobranchia.







Opisthobranchia (seaslugs) is subdivided in seven order, in agreement with the evolutionary history of the group : Cephalapsidea (headshield slugs), Sacoglossa (sap-sucking slugs), Anaspidea (sea hares), Notaspidea (sidegill slugs), Thecosomata (sea butterflies), Gymnosomata (sea angels) and Nudibranchia (nudibranchs).

They are highly evolved gastropods, characterised by a single gill behind the heart, from which the group derives its name (Greek : opistho-, behind; brankhia, gills) and two pairs of tentacles. The oral tentacles situated close to the mouth and behind them, the rhinophores which are olfactory organs.

They are principally soft-bodied marine creatures with a small or absent shell and no operculem.

Their bodies have undergone detorsion, an evolutionary reversal of the 180° torsion of their immediate ancestors (caracteristic of the Prosobranchia).

This detorsion has two principal consequences : gills behind the heart and noncross nervous system.