Nudibranchs - (sn Opisthobranchia)

Opisthobranchs are principally soft-bodied marine creatures with a reduced or absent shell and no operculum.
Their bodies have undergone detorsion, an evolutionary reversal of the 180° torsion of their immediate ancestors.
There is no marked distinction between head and mantle. The tentacles, situated close to the mouth, are used for orientation. Behind them you can find the rhinophores, olfactory organs often with complex forms. The middle part of the foot is the sole, used for locomotion. The sides of the foot have evolved into parapodia, fleshy winglike outgrowths. In several suborders, such as the Thecosomata and Gymnosomata, these parapodia are used to move in a swimming motion.
Many have brilliant colors and carry stinging cells, so that predators may learn to avoid them as a food source.