(phylum Mollusca, class Cephalopoda)
A subclass of cephalopods which have one pair of gills. The true shell, if it occurs at all, is internal. The head possesses eyes and a mouth that bears a pair of mandibles and a radula. Eight arms extend from the head. Some forms also have two additional, retractile tentacles. A hyponome is also present. There are four extant orders and one extinct order. The subclass first appeared in the lower Carboniferous.
Subjects: Zoology and Animal Sciences.