Nudibranchia of the world
Depicted: Okenia elegans, Hermaea bifida, Doto coronata, Facelina auriculata, Ancula gibbosa, Dendronotus frondosus, Tritonia hombergii.
“Nudibranch" literally means "naked-gill”, and among all the colorful and ornate sea slugs out there (the vast majority of which are not nudibranchs), there are very few non-nudibranchia that have these highly-exposed respiratory structures. The rosettes and side-plumes of these naked gills are present in all true nudibranchs, and are soft to the touch.
Many other sea slugs have “exposed” gills outside their body, but they are almost always keratinized into structures called cerata. When you encounter cerata in nudibranchs, however, they are not respiratory organs. They are envenomation structures, which store the nematocysts from food that the nuddies have consumed. When disturbed, they can release these second-hand stingers and inject the same venom found in their prey - which, by the way, often includes highly venomous and paralytic jellyfish and jellyfish relatives.
All images extracted from plate 43 of Kunstformen der Natur. Ernst Haeckel, 1904.