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.