Photograph by Carlos Villoch

(PHYLUM: Mollusca, CLASS: Cephalopoda, SUBCLASS: Nautiloidea)

The ancient ancestors of the octopus, squid and cuttlefish had external shells, just like other molluscs, for example snails, do today. The Nautiloidea first appeared in the Cambrian period (c.570 - 510 Million Years Ago) and were once, along with the now extinct ammonoids, major marine carnivores. Today, only a few shelled cephalopods survive, and the chambered Nautilus is the best known. This species, which grows to 200 millimetres in diameter, is found throughout the tropical west Pacific. The shell is divided into chambers, and the animal can control its buoyancy by changing the amount of gas and fluid in them.

Click here to view the 'Picture of the Week' archive.