Ceratae, that all Aeolidina have on their backs and sides, in addition to the aforementioned functions of digesting and defence, also serve as gills (branchiae), executing the main gas exchange.
Doridina, that do not have cerata, have their flower-shaped branchiae rear at their back. In fact, slugs of the order Nudibranchia were called "nude gilled" because of these openly swaying respiratory organs.