The Glaucus Atlanticus is commonly known as The Sea Swallow, The Blue Glaucus, The Blue Sea Slug, The Blue Ocean Slug, but I prefer The Blue Dragon. The Blue Dragon is a sea slug, and a marine gastropod mollusk in the family Glaucidae. Most people do not know of their existence unless they are washed up on the beach. It is one of the worlds rarest mollusks. There are even some people who say they should not exist, they should just be something somebody imagined. The normal size for a Blue Dragon is up to about 3 cm, depending on the animal's age.