The order of Anaspidae (Sea Hares) are part of Phylum Mollusca, class of Gastropoda and subclass Opisthobranchia (sea slugs). They are cousins to the more well-known Order of Nudibranchs. They are a more primitive version of sea slug and do not have exposed gills. They are slow moving, fast growing, larger sea slugs. They have both male and female sexual organs and when mating will mate by mounting from behind, often forming large chains when mating (where the animal at the end of the chain does not get pregnant).