Fam. 4. Limapontiidae.—Body slug-like, liver scarcely ramified, no branchiae, shell, or appendages. Genera: Limapontia, Actaeonia, Cenia.
Sub-order III. Nudibranchiata.—Shell absent in the adult, no ctenidium proper, or osphradium, cerata dorsal or dorso-lateral, nervous system concentrated, kidney not compact, ramified, penis retractile, jaws and radula usually present.
Section I. Cladohepatica.—Cerata usually latero-dorsal, elongated, or arborescent, buccal mass strong, jaws present, liver generally ramified, rami generally entering the cerata.
Fam. 1. Aeolidiidae.—Body slug-like, head with tentacles and rhinophores, dorsal area with rows of cerata, which usually contain sting-cells, radula variable. Genera: Aeolis, Cratena, Tergipes, Coryphella, Favorinus, Facelina, Flabellina, Fiona, Glaucus, Janus, Hero, with many sub-genera.
Fam. 2. Tethymelibidae.—Body slug-like, large, cerata very large, no sting-cells, head large, cowl-shaped, no tentacles, rhinophores much foliated, no radula. Genera: Tethys, Melibe. The cerata of Tethys, which are capable of independent movement when severed, have been described as parasitic worms. Tethys feeds on molluscs and Crustacea.
Fam. 3. Lomanotidae.—Body slug-like, dorsum prominent, undulating or lobed, with one row of small cerata, no tentacles, rhinophores much foliated, radula with uncinated dentate laterals. Single genus, Lomanotus.
Fam. 4. Dotonidae.—Body slug-like, small, two rows of cerata, each ceras surrounded by a ring of tubercles, rhinophores simple, radula uniseriate. Single genus, Doto.
Fam. 5. Dendronotidae.—Body slug-like, somewhat compressed, two rows of arborescent cerata, no tentacles, frontal margin with arborescent papillae, rhinophores arborescent, radula multiseriate. Genera: Campaspe, Dendronotus.
Fam. 6. Bornellidae.—Two rows of dorsal papillae, with branchiform appendages at the base, rhinophores foliate, radula multiseriate. Single genus, Bornella.
Fam. 7. Scyllaeidae.—Body oblong, compressed, two large foliated cerata with branchial appendages on the inner side, no tentacles, rhinophores large, radula multiseriate. Single genus, Scyllaea.