Nudibranch: This is the famous mollusc without a shell i.e. 'nudi' in Latin. Most of them have very bright colours, this type – Phyllidia in particular, to warn off predators. They move like a slug so they are always vunerable. Some species feed on the fire coral, Hydrozoa, and are able to transfer a part of the stinging cells into their own tissues. Others cover themselves in a pseudo-shell made up of the calcerous spicules of the sponges on which they feed. Still others have special glands that secrete repellent substances. Whatever their form they make great photographs for the snorkeller.