Nudibranchs are a group of sea slugs characterized by having no shell, thus having their gills exposed. The name in latin, nudibranchia means precisely that: nudi (nude, exposed) + branchia (gills).
Instead of the protective shell, the nudibranchs use other intricate defense mechanisms and that's one of the reasons they are such an interesting group to study. Not to mention their exquisite appearance.

More info about this species:

www.seaslugforum.net/flabisch.htm
hypselodoris.blogspot.com/2011/04/flabellina-ischitana-hi...