Fam. 1.—Tylodinidae. Shell external and conical; anterior tentacles form a frontal veil; ctenidium extending only over right side; a distinct osphradium. Tylodina.
Fam. 2.—Umbrellidae. Shell external, conical, much flattened; anterior tentacles very small, and situated with the mouth in a notch of the foot below the head; ctenidium very large. Umbrella.
Fam. 3.—Pleurobranchidae. Shell covered by mantle, or absent; anterior tentacles form a frontal veil; mantle contains spicules. Pleurobranchus. Berthella. Haliotinella. Oscanius, British. Oscaniella. Oscaniopsis. Pleurobranchaea.
Sub-order 2.—Nudibranchia. Shell absent in the adult; no ctenidium or osphradium. Body generally slug-like, and externally symmetrical. Visceral mass not marked off from the foot, except in Hedylidae. Dorsal respiratory appendages frequently present. Visceral commissure reduced; nervous system concentrated on dorsal side of oesophagus. Marine; generally carnivorous, and brightly coloured, affording many instances of protective resemblance.
Tribe 1.—Tritoniomorpha. Liver wholly or partially contained in the visceral mass. Anus lateral, on the right side. Usually two rows of ramified dorsal appendages. Genital duct diaulic; male and female apertures contiguous.
Fam. 1.—Tritoniidae. Anterior tentacles form a frontal veil; foot rather broad. Tritonia, British. Marionia.
Fam. 2.—Scyllaeidae. No anterior tentacles; dorsal appendages broad and foliaceous; foot very narrow; stomach with horny plates. Scyllaea, pelagic.
Fam. 3.—Phyllirhoidae. No anterior tentacles, and no dorsal appendages; body laterally compressed, transparent; pelagic. Phyllirhoë.
Fam. 4.—Tethyidae. Head broad, surrounded by a funnel-shaped velum or hood; no radula; dorsal appendages foliaceous. Tethys. Melibe.
Fam. 5.—Dendronotidae. Anterior tentacles forming a scalloped frontal veil; dorsal appendages and tentacles similarly ramified. Dendronotus. Campaspe.