The males and females of this free-swimming octopus look very different. The female lives in a white paper-thin shell (often called a 'paper nautilus') that reaches around 27 cm long. The common name of this octopus comes from the knobbed ridges across the sides of the shell. The female has a round body and eight arms, each with two rows of suckers. The first arm pair has two wide webs at the ends. These webs both secrete the white shell and are spread over the outside of the shell when the animal is swimming. The webs can change colour from silver to dark maroon red. The male is much smaller, being less than 3 cm long. It has no shell, a rounded body and large eyes. It has eight arms, each with two rows of suckers. The third left arm is much longer and highly modified. It develops in a pouch that reaches around the same size as the body. Female mantle length to 9 cm, total length to 27 cm, shell diameter to 27 cm. Male mantle length to 14 mm, total length to 3 cm.