Subclass Coleolidea contains the dibranchiate taxa of the class Cephalopoda whose respiratory system consists of a single pair of gills. The foot has fused with the head and by division it evolved into 8 arms fitted with suckers. Additionally, the cuttlefish (order Sepioidea) and squids (order Teuthoidea) have two long tentacles ending in a club provided with suckers. In some squids, suckers have evolved into hooks.
Cuttlefish and squids have suckers reinforced by a horny ring, which can be smooth or denticulate. The arms are numbered from the dorsal pair (1st) to the ventral pair (4th). The octopods and some squids have their arms connected by a thin membrane; the web. In some finned octopuses this wed extends almost up to the tips of the arms. Lost arms or tentacles can regenerate rapidly.
The mouth is located at the top of the head, in the centre of the arms crown. It is made of two strong horny mandibles and looks like a parrot's beak. Squids and cuttlefish possess a buccal membrane, sometimes provided with tiny suckers, which hold the arms together when the animal is swimming. On the ventral side, between head and mantle, the funnel is located. This is an important organ consisting of a conical tube narrowed at its anterior extremity and attached dorsally to the head and to the dorsal side of the mantle cavity by strong muscles. There is an aperture between the head and the mantle. When breathing the animal inhales water through the aperture. During exhalation, it closes the aperture and expels the water-along with any excrements-through the funnel.
The fins, attached to the mantle, vary in shape. The true octopus has no fins. When present, the shell is internal.