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:
Aplysiopsis enteromorphae graze happily on the filamentous green algae in the largest tidepool below the Visitor Center at Cape Perpetua. They are tiny blackish, grey, dark olive green. Watch your step as they are on average less than a centimeter and hang out on the very shallow shelf of the pool which is easily accessible by large feet.