Fam. 8.—Philinidae. Cephalic shield broad, thick and simple; shell wholly internal, thin, spire much reduced, aperture very large. Philine, British. Cryptophthalmus. Chelinodura. Phanerophthalmus. Colpodaspis, British. Colobocephalus.
Fam. 9.—Doridiidae. Cephalic shield ending posteriorly in a median point; shell internal, largely membranous; no radula or stomachal plates. Doridium. Navarchus.
Fam. 10.—Gastropteridae. Cephalic shield pointed behind; shell internal, chiefly membranous, with calcified nucleus, nautiloid; parapodia forming fins. Gastropteron.
Fam. 11.—Runcinidae. Cephalic shield continuous with dorsal integument; no shell; ctenidium projecting from mantle cavity. Runcina.
Fam. 12.—Lophocercidae. Shell external, globular or ovoid; foot elongated, parapodia separate from ventral surface; genital duct diaulic. Lobiger. Lophocercus.
The next three families form the group formerly known as Thecosomatous Pteropods. They are all pelagic, the foot being entirely transformed into a pair of anterior fins; eyes are absent, and the nerve centres are concentrated on the ventral side of the oesophagus.
Fam. 13.—Limacinidae. Dextral animals, with shell coiled pseudo-sinistrally; operculum with sinistral spiral; pallial cavity dorsal. Limacina, British. Peraclis, ctenidium present.
Fam. 14.—Cymbuliidae. Adult without shell; a sub-epithelial pseudoconch formed by connective tissue; pallial cavity ventral. Cymbulia. Cymbuliopsis. Gleba. Desmopterus.
Fam. 15.—Cavoliniidae. Shell not coiled, symmetrical; pallial cavity ventral. Cavolinia. Clio. Cuvierina.
Tribe 2.—Aplysiomorpha. Shell more or less internal, much reduced or absent. Head bears two pairs of tentacles. Parapodia separate from ventral surface, and generally transformed into swimming lobes. Visceral commissure much shortened, except in Aplysia. Genital duct monaulic; hermaphrodite duct connected with penis by a ciliated groove. Animals either swim or crawl.