Section SIPHONIDA—Mantle lobes more or less united to form tubular siphons.
Families—Pholadidæ, Gastrochænidæ, Anatinidæ, Myacidæ, Solenidæ, Tellinidæ, Mactridæ, Veneridæ, Cyprinidæ, Lucinidæ, Cardiadæ, &c.
Section ASIPHONIDA—Mantle lobes free or nearly so. No true siphons.
Families—Arcadæ, Mytilidæ, Aviculidæ, Ostreidæ, &c.
Class CEPHALOPHORA—Head-bearing. Usually enclosed in a univalve shell.
Section PTEROPODA—Wing-footed molluscs.
Section GASTEROPODA—Stomach-footed molluscs.
Order Nucleobranchiata—Viscera form a nucleus on the back.
Order Opisthobranchiata—Shell generally absent. Gills more or less exposed.
Section Nudibranchiata—Naked gills.