The Cephalopoda
Squids, octopuses, nautilus, and ammonites

		No. of described species: 700 (but more than 10,000 extinct species) First appearance: Middle Cambrian Habitats: marine, in the water column and on the sea floor Shapes: squids and octopuses and animals like these with shells Feeding types: carnivorous hunters

Cephalopods are thought to have evolved from monoplacophoran-like ancestors. Septa formed at the apex as the animal grew and withdrew into a newly formed body chamber. The old chambers are gas-filled and provide buoyancy for the organism. The foot was modified into a funnel that provided jet propulsion for movement.

Giant squid (Architeuthis) are the largest invertebrates, up to 21 m in length. The cephalopods include the largest living, as well as largest extinct, molluscs: ammonite shells range to over 2 m across and body sizes of living squid range up to 8 m with tentacles exceeding 21 m in length. The smallest cephalopods are around 2 cm in length.

Fossil record
Cephalopods were once one of the dominant marine animals but today there are only about 700 living species. In fact, more than 10,000 extinct species have been described from the fossil record. They appeared in the Middle Cambrian after other major molluscan groups, then quickly diversified. The most common fossil cephalopods are nautiloids and ammonoids — the soft-bodied octopuses and squids don't fossilize very well. Nautiloids have survived until today, but the ammonoids, one of the most common groups of fossils, went extinct with the dinosaurs at the end of the Cretaceous.

Cephalopods are much more variable in their diversity through time than other molluscan groups. They are hit by numerous extinctions (e.g., terminal Permian, Triassic, Cretaceous events), but typically showed rapid replacement (and subsequent radiation) by the survivors.

Life history & ecology
Cephalopods have an amazing ability to rapidly change color (using numerous chromatophores in the skin), body shape, and texture, all of which is under nervous control. Most cephalopods can swim using jet propulsion, the pulses generated by the muscular walls of the pallial cavity. Some also use undulating movements of paired fins at the distal end of the mantle for swimming. Many can expel a cloud of ink to create a "smoke screen" to assist escape. Tentacles (cephalic in origin) surround the mouth on the head and capture prey. They often bear suckers, sometimes hooks and a pair of retractile tentacles (arms) are found in some groups. All of these features make cephalopods extremely effective predators as well as giving them the ability to avoid being eaten themselves.

Perhaps the most interesting aspect of cephalopods is their incredible intelligence. With the largest brains outside of vertebrates and highly developed eyes and other sense organs, these animals are able to do many of the tasks we think of only as existing in vertebrates. Experiments with cephalopods have shown that they can learn, have good memories, and excellent powers of discrimination.

Cephalopods have a single gonad and separate sexes. The males produce spermatophores which are transferred to females following typically complex courtship, which may involve color changes. The spermatophore is transferred by the male using a penis (some squid, vampire squids, and cirrate octopuses) or (in nearly all others) a modified arm (hectocotylus). Nautilus uses four modified arms. Some taxa are sexually dimorphic. Fertilization is internal with egg capsules being laid and development is direct. Eggs are large and yolk-rich, and the embryonic development of cephalopods is different from that of all other molluscs. Cephalopods never go through a larval stage, they just develop directly into juveniles. Both the eggs and young may be brooded, benthic, or pelagic.

More on morphology
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 includes a pair of accessory hearts (except in nautilus). Cephalopods have powerful, modified jaws (beaks) and a small radula. 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. 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.

Systematics
Coleoidea, the squids, octopuses, and cuttlefish
Coleoids include the familiar squid and octopus as well as the less well-known cuttlefish. All coleoids have eight to ten suckered or hooked tentacles, a single pair of gills, and many have an ink sac. The two main groups are Octobrachia, which includes octopuses, paper argonauts, the pelagic cirrate octopods (Octopoda), and Vampire Squid (Vampyromorpha). These all have four pairs of tentacles and no internal shell.

The second group is Decabrachia — this includes the rams horn squid (Spirulida), cuttlefish and dumpling squid (Sepioidea), and common squid (Teuthoidea). All have four pairs of non-retractable arms, two pairs of retractable arms (tentacles), and most have an internal shell (reduced to a chitinous pen in squids). The extinct Belemnoidea also belong to this group.

Ammonoidea, the extinct ammonites
Ammonoidea was a large, diverse clade of extinct, shelled cephalopods that appeared in the Devonian and died out at the end of the Mesozoic. It was one of the most diverse and ecologically important groups of invertebrates ever. Impressions of animals suggest that they had 8-10 tentacles. It is likely that they looked somewhat like a squid with a curled shell.

Nautiloidea
The nautiloids include the chambered nautilus and its many fossil relatives. They first appeared in the Late Cambrian and underwent a rapid diversification in the Ordovician. All have a spiral nacreous shell with interconnected internal chambers. The head is covered with a hood and has numerous short, suckerless tentacles. There are two pairs of gills and, unlike coleoids, there is no ink sac.
 

Original text by Paul Bunje, UCMP.