Dibranchiata

Dibranchiata, the sub-class of Cephalopoda (q.v.) including all the living forms excepting the Nautilus. The name is given them because they all possess one pair of gills, whereas the Nautilus, and probably the others of the second sub-class, possess two pairs. The shell is very varied: it may be entirely external, as in the female Paper Nautilus (Argonauta), but is then not chambered; it may be chambered and only partly external as in Spirula, or chambered and wholly internal as in the Belemnites - in these latter the chambered portion is protected by a solid "guard" - or the shell may be a laminated plate, wholly internal, calcareous in the Cuttle Fish and horny in the Squids; finally, the shell may be absent as in the Devil Fish (Octopus). The animal has never more than ten arms, and in one group has only eight; these are always provided with suckers. The siphon is a closed funnel-shaped aperture; an ink sac is always present. The Dibranchiata are divided into two orders: the Decapoda (q.v.), including the Belemnites, Spirula, Cuttle Fish, Squids, etc., and the Octapoda (q.v.), including the Devil Fish, Paper Nautilus, etc. The class began in the Trias and has been since increasing in importance, while the Tetrabranchs have become almost extinct.