Cryptobranch dorid nudibranchs, previously known as the taxon Cryptobranchia, are nudibranch sea slugs within the clade Doridacea. These slugs are called “cryptobranch” meaning “hidden gill”, because they are able to retract their gills into a gill pocket, in contrast with nudibranchs in the traditional group phanerobranchs or the Phanerobranchia, which taxon is probably paraphyletic, in other words composed of more than one evolutionary lineage.(source)