Janolids have had a somewhat tortured taxonomic history having been shunted from Proctonotidae to Zephyrinidae to Janolidae and back to Proctonotidae again. A 2019 morphological and molecular study examined some species of Madrellidae and Proctonotidae and suggests the validity of reinstating Janolidae for certain genera. The general body shape of janolids is elongate, flattened and widest anteriorly. The notum carries many slender or swollen or even bulbous cerata which may be smooth or covered in numerous papillae or tubercles. A feature that readily distinguishes the Janolidae from the aeolid nudibranchs is that the cerata of the janolids, like the madrellids, also arise around the anterior margin of the mantle, forward of the rhinophores, forming an unbroken anterior fringe. The digestive diverticula often extend into the cerata (often lacking in the anterior-most cerata) and in most species are branched. The mantle is continuously fused with the broad foot which ends in a tapering tail. The anus opens on the dorsal midline in the posterior half, a feature that also serves to differentiate the janolids from the aeolids. The oral veil of janolids is characterized by the possession of short tentacular anterior corners of cylindrical shape. The relatively tall rhinophores cannot be retracted and are lamellate (perfoliate) in most but in some carry tubercles instead. The janolids also possess a ridge or caruncle between the rhinophore bases believed to perform a sensory function. There are no separate gill structures gaseous exchange being performed across the large surface area provided by the numerous cerata. Janolids feed upon bryozoans.