a) includes largest, fastest, and most intelligent invertebrates; purely carnivorous (eat fish, crustaceans, molluscs)

b) body plan modified for pelagic life; shell is reduced or absent in many; head and foot form tentacles and siphon

c) extinct forms had well-developed shells with gas-filled chambers

d) mantle is muscular and mantle cavity is large; jet-propulsion is achieved via antagonism of circular and radial mantle muscles

e) impressive eyes, balance organs (statocysts) and nervous system (largest brain in invertebrates, giant axons and stellate ganglia)

f) 2 pair (Nautilus only) or 1 pair (all others) of ctenidia lack cilia!

g) have a true 'closed' circulatory system lined by endothelium, with accessory hearts for gills

h) sexes separate; copulate with hectocotylus; unusual development

i) 3 subclasses: I) Nautiloidea (1 genus); II) Ammonoidea (extinct); III) Coleoidea (Or. Decapoda, Or. Octopoda) (based on number of ctenidia and arms/tentacles, and on presence/absence of suckers)