Sea slug is a common name for some marine invertebrates with varying levels of resemblance to terrestrial slugs. The often bright colors of reef-dwelling species implies that these animals are under constant threat of predators, but the color can serve as a warning to other animals of the sea slug's stinging cells. Most sea slugs have a pair of rhinophores—sensory tentacles used primarily for the sense of smell—on their head, with a small eye at the base of each rhinophore. Many have feathery structures on the back, often in a contrasting color, which act as gills. Sea Slugs even have brains!