There are six species of Nautiloidea; they have a snail-like shell which is divided by septa into gas-filled
chambers, with only the last chamber being occupied by the animal, seeNautiluspompilus, below. A fully grown adult may have as many as 30 chambers. The empty
chambers are buoyancy aids.
The septa are perforated in the middle, and through this a cord called the siphuncle passes (see the diagram below) and secretes gas into the empty chambers. The siphuncle is a cord of tissue running through the shell of the nautiloid connecting all the chambers with the body of the animal.
The mouth is surrounded by up to 90 tentacles or
arms; all suckerless.
The hood acts as a cover or operculum when the animal withdraws into the shell. However, when the hood is closed it cannot breathe, so this only occurs in an emergency. The eyes and nervous system are relatively simple
compared to the Coleoidea (see below), and the eyes have no lens.
All species are carnivorous.
Nautilus pompilus (right) is found in the Indo-Pacific region near the sea bed in waters up to 500 m deep. During the night it rises nearer the surface to feed. It is about 20 cm long when fully grown, and takes around 15 to 20 years to reach sexual maturity.
It swims backwards at about the same speed as a person swimming leisurely.