What is a Cephalopod?

Cephalopods The cephalopods are marine molluscs in which the head is surrounded by a ring of sucker-covered 'arms' or tentacles. The cephalopods include the octopus, which has eight arms, and the squid and cuttlefish which have ten. The group also includes the rare pearly nautilus and the extinct ammonites and belemnites.

Like the other molluscs, the cephalopods have soft-bodies which are partly enclosed by a thick fold of skin called the mantle. The mantle secretes an outer shell in most molluscs, but the only living cephalopod with such a shell is the pearly nautilus. The cuttlefish has a chalky 'cuttlebone' inside its mantle, while the squid has a thin horny plate called a pen. Most octopuses have no trace of a shell at all.

The cephalopod mantle is very muscular and it covers all but the head and arms of the animal. It is attached to the body just behind the head and it encloses the mantle cavity in which the gills are situated. Water is drawn into the cavity through small openings at the front, but it leaves through a short spout called the siphon. When one of these animals is disturbed, the mantle contracts strongly and forces a jet of water through the siphon. The effect of this is to shoot the animal backwards through the water. This efficient escape mechanism is often supported by a 'smoke screen' produced by squirting an inky fluid into the water.

The octopus is often regarded as dangerous but such a reputation is undeserved by the group. Most octopuses have bodies only a few centimetres across and they soon shoot away if a diver approaches. They spend most of their time on the sea bed and feed mainly on crabs and other crustaceans, which they catch with their arms and chew with their strong parrot-like beaks. However, the small blue-ringed octopus of Australia has venom injected by the beak which will kill a man. Cuttlefish also live mainly on the sea bed, although they are more active than an octopus and can swim slowly by flapping the edges of the mantle. Two of their arms are much longer than the others and are used to catch passing shrimps and other small creatures.

Squids are usually found in the open sea and are numerous. They feed on crustaceans and small fish, and in turn are eaten by larger fish. Most squids are only a few centimetres long, but the giant squid reaches a length exceeding 9 m. This creature is known from remains found in the stomachs of sperm whales or others washed on to beaches.

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