The cephalopods are an ancient and very successful group, some of which are now the most advanced of all the invertebrates.  They have long been among the dominant large predators in our oceans. Two groups of cephalopods exist today:  The Tetrabranchia (Nautiloidea) with a few species of pearly nautilus and the Dibranchia (Coleoidea), containing the squids, cuttlefishes, octopods and vampire squids (genera, anyone?).

The Tetrabranchia possess an external shell and two pairs of gills.  The Dibranchia either have a much-reduced internal shell or it is missing all together and they possess a single pair of gills.  From an evolutionary standpoint, the cephalopods appear to be a diminishing group, which is odd, considering the extreme state of intelligence of some species!   There are only about 400 species in existence now, compared to 10,000+ known fossil species.

        On the whole cephalopods are adapted for a free-swimming existence.  Some cephalopods such as the octopus, however, have largely given this up for a less active bottom dwelling life, although many of these are active hunters and scavengers.

     Cephalopods are the most active of all the molluscs and can even rival the fish for swimming speeds. Although there are relatively few species of living cephalopods, they inhabit a great variety of habitats in all the world's oceans.

     Most cephalopods have a sac that produces brown or purplish-black ink used to ward off attacking fish or to serve as a protective smoke screen.

     Cephalopods have attained the largest size of all the molluscs.  Most range between a few inches (5 cms.) to several feet (2- 4 meters) in length; however, the giant squids (Architeuthis) can attain a total length of about 55 to 60 feet (20+ meters), although the possibility of far larger individuals lurking in the ocean's depths has always been the subject of sea-lore, legend and speculation - these are the famous, greatly-feared sea-monsters called Kraken by the Norse, and various respectful names in many other cultures.  Individual species, especially the squid, are often very abundant and provide major targets for marine fisheries. The chalky cuttlefish bone is also used for a calcium supplement for birds and at time used in making toothpaste.

     Only the tetrabranchs (nautilids) produce an external shell in this class of molluscs.  Externally, the shell of the nautilus is creamy white with broad reddish-brown stripes. Inside it is brilliant, iridescent mother-of-pearl. The nautaloid shell is very complex, chambered and spiraled over the head of the animal.  Even though coiled, it is radically different than that of the gastropods, being divided by transverse septa  (diagram) creating internal chambers.  The animal only inhabits the last chamber.  As the animal grows, it periodically moves forward, and the posterior part of its mantle secretes a new septum.   The septa are perforated in the middle.  Through this opening, a chord or tube of body tissue called the siphuncle extends from the visceral mass of the animal to the tip of the most distal chamber.  The siphuncle secretes a gas into these empty chambers, making the shell more or less buoyant allowing the animal to rise up or sink down further in the water column, as the animal needs to.

     Not all fossil cephalopods have coiled shells, nor are the shells of all coiled species similar in shape to that of the nautilids.  The cephalopod ancestors probably had a straight shell shaped like a cone as is shown in the earliest fossil records. The most famous fossil nautilids are the Ammonites, and many people collect and pay surprisingly high for their remains, many of which still bear some of the original shell material, preserved for up to 200 million years!! Some of these exceed 8 feet (3 meters) in length with an opening of one foot (31 cms.) in diameter.  In some species the shell is so loosely coiled that the whorls are often unconnected, although most are compactly coiled.  The largest coiled fossil belonged to a critter with the unwieldy moniker of Pachydiscus seppenradensis that measured up to a little over 8 feet  (just under 3 meters) in diameter.  The smallest yet found is less than an inch (2.5 cms) in diameter.

     The argonauts produce an egg case that many people confuse for a shell.  (See reproduction section for more on this brooding chamber).

     External shells are absent on the Dibranchia.  The squids have an internal pen the octopus have lost the shell all together (homologous with shell).  The oldest known Dibranchia are the fossil Belomoids.  The internal shell here was much thickened at the apical and lateral walls and in some the anterior dorsal wall projected over the viscera.

     In the Spirula, a common worldwide deep-water squid, the shell is internal.  It is a loosely coiled tube divided by septa into small chambers.

     All cephalopods swim by rapidly expelling water from their mantle cavity.

     The squids are highly specialized at swimming, and bear a pair of posterior, lateral fins that act as stabilizers.  The mantle contains both longitudinal and circular muscle fibers.  On inhalation, the circular muscles relax and the longitudinal muscles contract enlarging the mantle cavity.  This creates a suction and pulls in water through the space located between the anterior edge of the mantle and the head.  When the mantle cavity is full the increasing pressure created by the water causes the circular muscles to contract that in turn closes the opening through which the water entered.  The longitudinal muscles then contract and force the water through the ventral tubular funnel.  The force of the expelled water as it leaves the funnel propels the animal in the opposite direction.  This funnel is also quite mobile, allowing the animal to maneuver either backwards or forwards.   Speed depends on how forcibly the water is expelled.  Squid are able to hover or dart away very quickly.  In normal swimming, the arms are stretched anteriorly and held closely together.  Their fins are extended and they undulate gently.  These fins wrap tightly against the body during rapid swimming. When escaping from predators such as large fish and dolphins, squid often jump right out of the water, and some species can travel surprising distances in these jumps, their lateral fins acting as little "wings"!

     Nautilus can also swim with surprising speed.  The process for this action is the same as that in the squid; however, the ejection of the water is produced when their body and funnel muscles contract rather than those of the mantle.  Nautilus often rest on the bottom with their tentacles forming a stabilizing platform. Whether swimming or resting, their gas filled chambers keep their shell upright at all times.  Scientists have yet to discover just how the nautilus regulates this gas production: perhaps you could be the one to discover this!!

     The octopus is built for a more sedentary life style.  Their body is globular and bag-like and it has no fins.  The mantle edges are fused dorsally and laterally to the body walls producing a much smaller aperture into the mantle cavity.    Octopus are able to swim as the squid do but in a jerkier fashion; however, they prefer to crawl about the rocks and crevices on the ocean floor.  Their arms, which are studded with sucker discs, are used to pull the animal along or to anchor it to the substratum.   Some octopods, the Vitreledonellidae, have returned to a swimming existence.  Their arms have become webbed and look somewhat like an umbrella.  These web arms are then used similarly to how a human swimmer would use his arms while doing the breaststroke.

     The circulation of water through the mantle not only produces the power for swimming, but it provides oxygen for their gills.  The Tetrabranchia have four gills and the Dibranchia have two. The surface area of cephalopod gills have been much increased by a type of folding (more info needed here), and are not ciliated as in other molluscs. These cilia are unnecessary, as cephalopods are predators not filter feeders.   The circulation of water over the gills is the reverse of what it is in the gastropods.  Since water leaves the mantle cavity by means of the funnel, the exhalent current is ventral to the inhalent current.

     The circulatory system is largely closed in all cephalopods.  It has an extensive system of vessels making it the most complex and effective system of all the molluscs, enabling them to be much more intelligent and able to move rapidly and over extended periods of time.

     The circulatory system of an octopus is closed and consists of one systemic heart and two brachial hearts, two brachial glands (gills) and the blood vessels.

     The two branchial hearts, which sit at the gill base, collect unoxygenated blood from all the body parts.  They then contract and send this unoxygenated blood coursing throughout the capillaries of the gills.  The two auricles of the heart then drain the now oxygen rich blood from the gills and pass it into the medial ventricle.  The ventricle then pumps the blood out to the body via an anterior and posterior aorta, and eventually through smaller vessels and finally into tissue capillaries.

     Cephalopods have a cartilaginous brain case and a well developed nervous system.  They exhibit complex and very advanced behavior, which includes problem-solving and even curiosity!  (See article Octopuses are Smart Suckers!)

     Octopuses have the most complex brain of all the invertebrates.  They have long and short-term memories. They learn to solve puzzles quite quickly.  Once learned, they can rapidly solve the puzzle upon repetition and they remember how to do it in the future.

     All the dibranches (i.e., octopi, squid, etc.) have complex image-forming eyes that are comparative to a human's; focusing is done, however, by moving the lens in and out rather than by changing its shape as in the human eye.  (Diagram)

     The eyes of the Tetrabranchs (nautilids) are much less complex.  They are simple open pits at the end of short stalks. They lack the complex image-forming apparatus of the other cephalopods.

     The presence of the pedal and brachial nerve ganglia innervating the tentacles of the cephalopods is direct evidence that the tentacles of cephalopods are homologous to the foot in other molluscs.

     Tactile sense in all cephalopods is quite well developed.  Their sense of touch is most acute at the rim of each sucker. Studies have shown that even when blindfolded, an octopus can differentiate between objects of various sizes and shapes, often with remarkable accuracy:  Some can even detect size differences in spheres, for example, which most humans can't!

     Statocysts (organs of balance) are found in both tetrabranchs and disbranches.  They are large and particularly well developed in the dibranchs and are located imbedded in the cartilage present on each side of their brain.

     When threatened, they will often try to escape by releasing a cloud of purple-black ink (this ink is toxic) to confuse the enemy. The octopus can release several jets of this ink before its ink sac is empty.

     All cephalopoda, except the Nautilus have chromatophores imbedded in their skin.  The rapid expansion and contraction of these cells cause coloration changes to occur in the skin.  Tiny muscle cells cause the chromatophores to flatten out and widen out to a flat plate when they contract.  When these fibers relax, the chromatophores close and the pigment becomes concentrated.  Some species possess chromatophores of several colours (blue, purple, pink brown and black).  These colour changes can be very rapid.

     The octopuses (octopi, if you want to get "picky"!!)  are the masters of these rapid colour changes.  Changes are usually initiated by visual input.  In the octopus, the chromatophores consists of three sacs containing different colours.  These are activated and the colours are adjusted until the octopus is camouflaged.  Colouration, scientists now believe, also indicates an octopus  mood. Normally brown in colour, when they turn white they may be showing fear and when they turn red it may indicate that they are angry  (just like people!!).  This gives the cephalopods the ability to communicate by using colour changes.

     Cephalopods are all carnivorous.  A radula is present, but even more important is the pair of powerful beak-like jaws that enable the animal to bite and tear apart their food.

     The squid prey on fish and various invertebrates. When going after fish, squid dash in, grab a fish, pull it in and bite a triangular chunk out of its neck, thus severing its nerve cord.  Squid possess ten arms arranged in five pair around their head.  The eight short heavy appendages are called arms.  The other two, which are twice the length, are called tentacles.  The inner surface of each arm is flattened and covered with stalked, cup-shaped adhesive discs that function as suction cups.  Often hooks are also located around the periphery of each disc.  Attached to the floor of each suction disk are muscle fibers.  When a squid touches its prey with these suction cups, the muscle fibers contract and cause suction.    The two long tentacles are very mobile and not flattened until their end.  The spatulate ends bear suction discs and are used for grasping prey and drawing it to their mouths.

     The octopi dwell on the ocean floor and they usually just wait for their favorite meal of shrimps, crabs fish and other molluscs to venture within their reach. Octopods have eight tentacles, of equal length, covered with sessile suckers.  They do not possess a horny ring or hooks on their suckers, as do the squid.  The octopus wraps his tentacles around its prey and with suckers firmly attached, he pulls the prey to his parrot-like beak (mouth), where it is immobilized (see below), then torn apart and devoured.  Many octopi are active, intelligent hunters and scavengers that will eat nearly anything, and are VERY quick!!  This is why they tend to end up as the only inhabitants of fish tanks they are accidentally or innocently deposited in.

     The nautilids possess approximately 94 tentacles that arise from lobes that are arranged in both the inner and outer circle around their head.  These tentacles lack sucker or adhesive discs.  They are instead annulated and can be drawn into a sheath. Located above the head and tentacles is a leathery hood which can be pulled over the vulnerable soft body parts, which the animal pulls into its shell when, threatened.

     The beak is located in the buccal cavity and is used to bite and tear off large pieces of tissue.  These morsels are then shredded and drawn into the buccal cavity by means of the radula (think of the chain on a chainsaw going around).  The squid and octopus possess two pair of salivary glands that empty into the buccal cavity.  The one pair of glands secretes a digestive enzyme while the second pair produces a toxin. This poison enters the prey through the wound inflicted by the beak bite.

     The stomach is very muscular and a large cecum is attached at the anterior.  In Loligo the cecum is straight.  In Sepia, Octopus, and Nautilus it is spiraled.  The digestive glad in cephalopods is divided into a small, semi-distinct portion, sometimes referred to as a pancreas.  This pancreas constantly produces a digestive enzyme, which is stored in the cecum.  They also possess a large liver  which is roughly homologous (i.e., it serves similar functions) to the digestive gland in the other molluscs.  Hepatic secretions only enter the stomach during feeding.

     When food enters the stomach, both pancreatic and hepatic secretions are poured into the stomach via a groove in the cecal wall.  The food is churned and mixed with these digestive enzymes - this is how digestion begins, in the stomach.  This partly digested slurry is then passed into the cecum where it is completed.  The anterior (front) walls of the cecum contain an elaborate series of spiral ciliated folds, which separate nondigestible particles from the digestible cecal contents, which are retained, while the indigestible material is passed on to the intestine.  Absorption takes place entirely in the cecum and the residue left over is also passed into the intestine.  The straight or coiled intestine then carries this waste to the anus, which opens into the mantle cavity.

     The two nephridia characteristic of both the squid and octopods follow the basic molluscan design.  They are, however, rather compact and sac-like.  The reno-pericardial canal  (blood vessel from the heart (cardia) to the kidneys (the "reno" part)) has become enclosed within this sac.  The pancreas is also embedded in the sac and plays a role in waste removal. Through the middle of each kidney passes a vein surrounded by glandular tissue.  This tissue picks up the waste from the blood and deposits it into the sac cavity.  The nephridiopores open into the mantle cavity near the anus.   The siphon, which is attached to the underside of the head, extends into the mantle cavity and through this funnel body's wastes are expelled. (The cephalopod's eggs and ink are also expelled through this funnel)

     The nephridia of the nautilus are somewhat different.  They possess four nephridia and they have lost all connection with the pericardial cavity.

     Cephalopods usually are usually dioecious (two sexes), and fertilization is internal. Courtship, which in some species can be quite elaborate, is often a precursor to copulation.  Gonads are located in the posterior of the body.

     In some species, males can be distinguished by the modified sucker discs found at the tips of his longer two tentacles.  The male uses these long arms to remove a sperm packet from his mantle cavity and to then insert this packet into the female s mantle cavity.  This arm is autonomous and sometimes can get broken off while mating and is retained within the female s cavity.  If this happens, the male simply grows a new one: no problem!!

     Within two months of mating, the female octopus will attach long strands of clustered eggs (resembling a cluster of grapes) to the ceiling of her lair. While the eggs are incubating, the female will gently caress them to keep them clean and free of bacteria. She also keeps a steady flow of fresh, oxygenated water flowing over her precious eggs. When they are ready to hatch, her caresses become more rigorous, which helps the young to escape from their egg sacs.

     The pelagic Cephalopod family Argonautidae, commonly know as the paper nautilus, have a remarkable adaptation for egg deposition.  The two dorsal arms of the female are greatly expanded at their tip to form a membrane.  The expanded portion of each arm secretes one half of a beautiful calcareous bivalved shell. She then deposits her eggs directly into this case.  The shell acts as both a brood chamber and a retreat for the female.  The posterior of the female usually remains in this shell.  The male, which is much smaller than the female, does not have such a shell and is often found in cohabitation within the same shell, more or less as a freeloader! (Photo)