• Norwegian: bakgjellesnegler


In all sea slugs (formerly known as Opisthobranchia) you will find the gills behind the heart. The sea slugs originate from the sea snails which have the gills in front of the heart. The shell is usually internal or completely lost at some point during evolution. The name Heterobranchia means "different-gilled snails".


This subclass includes terrestrial and freshwater species too. The marine species, more specifically the sea slugs, are organized in several orders. Three of them are presented here: