Key to the Orders and Sub-orders of Cephalopods

A. Shell internal or absent; arms, eight to ten; gills, two.

Order DIBRANCHIATA

B. Arms eight, all alike; suckers fleshy; shell absent.

Sub-order OCTOPODA

Eight-armed Cephalopods

The Argonauts

The Devil-fishes

BB. Arms ten, two elongated; suckers with horny rims; shell internal.

Sub-order DECAPODA

Ten-armed Cephalopods

The Spirula

The Cuttles

The Squids

AA. Shell external; arms, more than ten; gills, four.

Order TETRABRANCHIATA,

The Chambered Nautilus