Smallest of the pterotracheids; maximal body length about 40 mm (He 12a). Terminal visceral nucleus, followed by a very short, ventral tail and either a permanent egg string (in females) or a filamentous extension (in males). Body highly transparent (the only pterotracheid that lacks pigmentation), with a thin cutis and without tubercles. Prominent tentacles present in males; lacking in females. Radula easily distinguished from that of other Pterotracheidae by the shape of the central tooth, which bears two long, sickle-shaped and pointed posterior processes on its basal plate. This species is the most transparent of all heteropods; it is very difficult to pick out from a fresh plankton sample. It has a circumtropical distribution and is extremely abundant in all oceans; sometimes equal in numbers to Atlantidae (Tesch, 1949).