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Cephalopods are the most complex and motile of the non-vertebrate metazoans, and show numerous
modifications of the general molluscan body plan. The chambered nautilus has an external shell,
but all other living cephalopods have either a reduced and internalized shell or
none at all. The calcareous shell of cuttlefish (the cuttlebone) is internal, as is that of the
rams horn squid, but other squid have the shell reduced to a horny pen and octopuses lack a
shell completely. The shells of cephalopods (other than the very reduced "pen" in squids) have
gas-filled chambers that assist with buoyancy.
Their highly developed, efficient circulatory system differs from that of other molluscs in being closed and including a pair of accessory hearts (except in Nautilus). Some cephalopods have paired fins at the distal end of the mantle for swimming. Tentacles (cephalic in origin) surround the mouth on the head for capturing prey. They often bear suckers, sometimes hooks and, in some groups, a pair of retractile tentacles (arms) are found. |
Cephalopods have powerful,
modified jaws (beaks) and a small radulae. There are large salivary glands in some
squids and octopuses that can produce highly toxic venoms for capturing prey. The muscular stomach
mixes the enzymes and food and passes
the semi-digested contents to a large caecum where ciliated leaflets sort the particles.
The nervous system is highly advanced with three major ganglia concentrated to form a large, efficient brain. Coleoid cephalopods also have two large stellate ganglia on the mantle that control both respiratory and locomotory functions of the mantle. These large brains are well-used: experiments on cephalopods have been shown that they can learn, have good memories, and excellent powers of discrimination. Their eyes are by far the most advanced in the invertebrates and are strongly convergent on vertebrate eyes. They are capable of resolving brightness, shape, size, and orientation. Other sensory structures include statocysts and olfactory organs for smelling. |
