What is "clionoidea"

Clionoidea is a taxonomic superfamily of sea slugs, specifically naked (i.e. unshelled) pteropods, marine opisthobranch gastropod mollusks in the clade Gymnosomata. They are sometimes called "sea angels" or "Naked Sea Butterflys" along with the other superfamily in the Gymnosomata. They can be found anywhere from the surface, to a depth of 350 meters. They are transparent and small, with the largest of the species being up to 5 cm.