Nudibranchs are colourful sea slugs whose name means "naked gills," derived from the Latin word nudus (naked) and the Greek word βρανχια (brankhia, meaning gills). This name reflects their external respiratory organs. They are soft-bodied sea slugs belonging to a class of mollusks called gastropods. Unlike other mollusks, nudibranchs have completely lost their shell and operculum as adults. Found all around the world—from the Arctic to tropical regions and even Antarctica—these marine animals are extremely diverse in shape, size, color, pattern, and form. There are over 3,000 known species, with many more yet to be discovered.