rectum also opens in the mantle cavity via the anus and the pair
of renal organs each opens into the mantle cavity via a renal
pore. The pen is an internal skeletal rod made of chitin. This
extends along the length of the body along the back of the
squid. It protects the internal organs on the upper surface. The
pen expands into a chitinous shield (pro-ostracum) at its
anterior end, to which the mantle tissues are attached.
The gills, suspended from the body wall by the suspensory gill membranes, are bipectinate - they have filaments on both sides of their axis. An extra heart, the branchial heart, is present at the base of each gill, ensuring that they have an efficient blood supply.
The ink-sac discharges via its own duct, which runs along the rectum, just behind the anus and is a black ovoid sac.
The renal sacs open either side of the anus. There is also a single genital opening on the left, just behind the renal openings.
Above: a diagram of a dissection of a male squid (Loligo) with a median cut in the ventral side of the mantle to reveal the organs of the mantle cavity. The cartilages (and corresponding sockets that they fit into) constitute the 'resisting apparatus' which seal the mantle when 'valve' muscles contract. The funnel can be directed, performing steering movements.
Mechanisms in Cephalopods
Squid, like some other marine animals can rely on speed of movement to keep them afloat, or they can achieve neutral bouyancy - that is they can have more-or-less the same density as sea-water so that they able simply to float. To achieve perfect neutral bouyancy is difficult, proteins are dense and make animal tissue denser than water. An active animal needs plenty of muscle protein which makes its body denser, but then it can swim when needed to readjust its height as it sinks. Some animals opt for a very different strategy, they have lower protein-content muscles which are more watery and less heavy.
These animals might not have the power to be continuously active, but then they don't need to be - they may move in short bursts when catching prey or wait in ambush for prey to find them. There is another complication, however, muscle protein quite severely impedes the diffusion of oxygen through tissues. However, not all swimming muscles need a rich oxygen supply - the white muscle which makes up the bulk of the body in bony fish is anaerobic muscle used in sprinting, it is strong and fast but fatigues quickly and has a poor oxygen supply. This kind of muscle is used in burst swimming or sprinting, as when catching prey and avoiding predators and so does not need to be used continuously for any length of time. The red muscle of bony fish has a rich oxygen supply and although much smaller and less powerful than the white muscle, the red muscle is able to move continuously and is used for cruising.
Fish with low protein content are often deep-sea fish that dwell at abyssal depths where food is scarce and they will wait in ambush, often luring prey with bioluminescent lures meant to resemble the prey food of the intended victim. The giant squid, Architeuthis, has a dense muscle structure indicative of an active animal. However, the story is not so simple. The giant squid uses another technique (used by many other sea creatures) to reduce its density. Its tissues have a high ammonium ion content and low sodium ion content - ammonium is replacing sodium (both ions have a single plus charge and so have similar properties). Ammonium has a much lower relative atomic mass than sodium and so, it is reasoned, tissues with a high ammonium/sodium ratio have low density. Indeed they do, but for a slightly different reason - it is not so much ionic mass that matters, but the way in which the ions interacts with water molecules around it as this has the greater effect on density of the final solution. A solution of ammonium ions is less dense than a solution of sodium ions of the same concentration. Interestingly, the giant squid has a much higher ammonium/sodium ratio (about 2) in its mantle than in its head and arms. This suggests that it hangs with the head and tentacles angled downwards, perhaps passively fishing for food that passes beneath it. This suggests that it might be a passive ambush hunter.
Cuttlefish resemble squid (and are often referred to as 'squid') but are quite different - their tail fin is extended forwards laterally to form an undulating skirt around the whole mantle, allowing these cephalopods to hover in a way that seems almost effortless and is quite fascinating to watch. Cuttlefish also have internal floats in the form of cuttlebone. The cuttlebone is a porous calcareous structure that fills much of the body of the cuttlefish and is a gas-filled internal shell that gives the cuttlefish buoyancy. This float will implode at around 200-600 m, so cuttlefish are shallow-water creatures largely confined to the continental shelf. The internal shell of squids is the skeletal pen (gladius) made of chitinous material and has no special function in buoyancy.
giant squid, Architeuthis dux, was denied by science for
a long time, despite the fact that many seamen had reported
seeing such creatures (kraken) and also despite the fact that
some tentacle remains had been pickled in a museum collection.
When it was finally ascertained that these creatures really
existed, the question remained as to how big they could grow.
Specimens are known in which the mantle length is over 2 m. The
arms and fishing tentacles add to their length, for a total
verifiable maximum length of about 13 m for females and 10 m for
Recently, the even larger colossal squid was realised to exist (although tentacles had been known for some time, it was not until 2003-2007 that the full size of these creatures was realised). This leads naturally to the question: how large is the largest squid? Studies of the stomach contents of sperm whales, which feed on giant squid (and being some 100 times heavier than the colossal squid can probably manage even the largest squid) which yield squid beaks, suggests that giant squid do not commonly exceed these proportions. This, however, still does not rule out the possibility of their existing rare mega-giants. Giant cephalopods were dominant predators in the Ordovician seas, some 450 million years ago. How big was the largest cephalopod of all time? This question remains unanswered.
The Nautilus is the only living cephalopod to retain an external shell. Many ancient and extinct cephalopods, like the Nautiloid shown below, had shells. The Nautiloids, some of which had shells up to at least 8 feet in length, had either straight conical shells, coiled shells or shells that were partially coiled like the one pictured below. Like the Nautilus the rear chambers of the shell were probably gas-filled for flotation.
Cephalopods, like many marine creatures, are equipped with lights. These lights have a number of functions:
Counterillumination: an animal that lives near to the surface of the sea will be well-hidden if it stays dark when seen from above, against the dark depths, but will appear dark if viewed from underneath against the downwelling light. To counter this, many fish and squid have dark backs but illuminated undersurfaces. Some squid are translucent, but their eyes and ink sacs are opaque, so they may have downward-pointing lights positioned underneath their eyes and ink sacs.
Communication: female squids tend to have lights at the tips of their arms which are presumably used to signal to the males. Squid may have complex species-specific patterns of lights on their bodies, and this may aid species identification in shoal formation or in other forms of communication.
Prey-capture: waving or flashing lights around to mimic smaller bioluminescent organisms is a good way to attract food. A smaller fish or crustacean swims toward the lights, expecting a feast, but is eaten instead!
Defense: a sudden flashing of lights can distract, confuse and possibly temporarily blind an attacker in the dark depths. Some squid can release luminescent ink (which may be in addition to the ink proper which is used in well-lit waters) as a decoy.
Path-finding: some animals use their lights to see where they are going! This is especially handy if your lights work at a wavelength that you can see but your potential prey cannot! Some fish exploit this mechanism in hunting.
Light-producing organs are called photophores. Photophores in cephalopods vary widely in design and a single animal may even have several different types. However, a fairly typical squid photophore is shown in section in the diagram below:
from Pringgenies and Jorgensen, 1994).
This photophore contains a lens (made of modified muscle cells) to focus the light beam. A reflector intensifies the light. This reflector is made up of a stack of very thin plates or lamellae, spaced 200 nm apart from one another. This reflects and diffracts the light in such a way as to intensify it by constructive interference. The central chamber contains the light-emitting tissue, in this case canals and pockets of tissue which house masses of bacteria. These bacteria are of a specific type and they produce the light. A canal, which opens into the mantle, circulates a current of water through the organ and through the interconnected chambers, by means of tiny beating hairlike cilia. This presumably provides the bacteria with oxygen and perhaps other substances.
Most cephalopods (excluding Nautilus and some octopuses) squirt ink as a defense mechanism. The ink sac is situated between the gills in the mantle cavity and ink is released into the exhalent jet of the siphon. The first tactic is to release large quantities of ink whilst jetting rapidly away (backwards). This generates a smokescreen to cover the cephalopod's escape. It is also possible that the ink contains chemicals that numb the sense of smell of the attacker (?). The second tactic is to generate decoys - less ink is released along with lots of mucus to entrap it. This creates a dark mass roughly the same size and shape as the squid and predators often mistakingly attack the decoy. The squid may release several such decoys, firing them to a small distance away from itself, and changes colour (turning pale) as it does so, adding to the attacker's confusion, and then attempts to escape. Some cephalopods release curtains of 'luminous ink' - light-emitting fluid when in the dark depths. Some can release both dark ink in well-lit waters and luminous ink in dark waters. The luminous 'ink' is released from a separate chromatophore gland rather than by the ink sac.
Arms/tentacles and Suckers
Squid and cuttlefish have ten 'tentacles' - 8 arms and two fishing tentacles, whilst the octopus has the 8 arms only. The arms/tentacles are made of solid blocks of muscle and connective tissues - there are no skeletal parts, not even the fluid-filled chambers of a hydrostatic skeleton. Instead, groups of muscles oppose (antagonise) one-another - longitudinal muscles shorten the tentacles, circular and transverse muscles make them narrower and more elongated, while helical muscles are presumably responsible for twisting movements. Thus, the muscles serve as both motors and in skeletal support.
The arms and the tentacle clubs possess suckers and/or hooks. The suckers may be toothed and in some squid they are replaced by hooks, or both hooks and suckers may be present. In octopus there are suckers but no teeth or hooks. In squid and cuttlefish the suckers are borne on stalks, whilst in octopus they are stalkless, although the base may extend to twice its normal length. The suckers can be rotated and tilted. The structure of an octopus sucker is shown below (redrawn from Kier and Smith 2002):
sucker is divided into three parts - the sucker itself or
infundibulum (I) and the sac-like acetabulum (A) and the mobile
base. The acetabulum wall (AW) and wall of the infundibulum
contain radial muscle fibres (R), circular muscle fibres (C) and
meridional muscles (M). these structures are enclosed in a tough
outer connective tissue sheath (OS). Connective tissue fibres
also criss-cross in the acetabulum wall. A pair of circular
sphincter muscles (S) - one large and one small - control the
diameter of the narrow orifice between the infundibulum and
acetabulum. Extrinsic muscles (E) and extrinsic circular muscle
(EC) ensheath the acetabulum. D, dermis; Ep, epithelium.
sucker secretes mucus, allowing it to create a water-tight seal
against a surface when suction is applied. The suckers of
cephalopods are capable of generating 1-3 atmospheres of
pressure and are also highly dextrous, being able to pass food
along the arms, for example. The most powerful suckers seem to
belong to fast-swimming squid, presumably since these catch
fast-moving prey. The tentacles are rapidly extended and if the
strike is successful then the clubs will hit the target and the
suckers will attach. The tentacles of the giant squid can be
over 10 m in length and are tipped with 10 cm diameter suckers.
In small squid, the speed of tentacle extension during
prey-capture has been filmed at over 2 m/s. Octopus will use
their webbed arms for gliding and for walking along the sea
bottom, as well as for food capture and to manipulate objects.
arms of octopuses can be detached at a predetermined weak point,
should an arm become trapped and the arm then slowly
Is this an alien from outer space? Not quite, its a 3D computer
model (Pov-Ray) of the Google-eyed glass squid, Teuthowenia pellucida. This squid occurs at
depths of 1600-2500 m (adult stage). When threatened, these
normally narrow squid inflates with water. If the threat
persists then they retract their head and tentacles and if this
is insufficient, then they fill the body cavity with ink,
causing the squid to apparently disappear in the darkness. These
squid can also use jet-propulsion to escape from danger.
Glass squids come in a remarkable variety of forms and some are so fragile that they are very hard, if not impossible, to capture intact by conventional means and so are poorly studied, but observations of the animals in their natural habitats are adding valuable data.
With their large heads and large eyes, cephalopods look intelligent and indeed they are. They probably
have the most complex brains among the invertebrates and some octopus are known to collect a tool
for later use - they will pick-up half a coconut shell and carry it with them until they find another half and
then they will make a shelter from them. This is a recent discovery, but there are old fables of
octopuses crawling up onto the beach at night and climbing palm trees to steal coconuts (presumably
for food) before scurrying back into the sea when approached in the morning! True or not, these old
stories are certainly possible. Octopuses have also been observed collecting and depositing rocks at their nest entrances to narrow the entrance if it is uncomfortably large and insecure.
Octopus is almost certainly the most intelligent of the
cephalopods. individual suckers are capable of fine precision
grip, folding to act like a pair of pincers which allow fine
tasks such as untying surgical silk. Suckers can also form
'conveyor belts', passing food from sucker to sucker towards the
mouth and empty shells in the reverse direction. they are also
involved in grooming and are very efficient at removing
parasites from the octopus integument. Controlling 8 arms with
suckers capable of such well-coordinated movements requires
considerable computational power and more than half of the
octopus' neurones are outside the brain and most of these are in
the arms. A nerve cord extends from the suboesophageal ganglia
down each arm and a circular nerve ring connects the 8 cords
together, to coordinate their movement when needed. Each such
nerve cord consists of a chain of connected ganglia and
additional sucker ganglia control suction cup movements. It is
thought that the fine coordination of the octopus' arms may be a
subconscious affair, much as learned motor skills are in humans,
such as when a skilled pianist plays a piano.
cephalopods exhibit social behaviour, they have intelligent
brains and they have suckered arms capable of complex
manipulations, so why have they not evolved civilisation? One
possibility is their lack of longevity. Cephalopods tend to die
after spawning and so are short-lived. Even captured giant
squids are only about 5-6 years old (a phenomenal growth rate!).
Perhaps these creatures never live long enough to make
discoveries and then pass them on to their offspring, indeed
they die before their young grow and develop.
Nevertheless, the possibilities have engaged the human imagination for decades. Cephalopods
inspired H. G. Wells' Martians and hence probably the famous Daleks, as well as the Illithid mindflayers
of Dungeons & Dragons. In all cases these aliens were super-intelligent. H. G. Wells added a final twist
in speculating that the Martians evolved from creatures more like us, humanoids with bones, but having
adapted to pushing buttons and pulling levers on a low-gravity world they lost their limbs, with the two
hands remaining as paired clusters of tentacles on either side of the face, and the bones disappearing.
They had classic cephalopod looks, however, with snakelike tentacles and large, luminous and disc-like
eyes! Similarly the Daleks evolved from a race of humans by a combination of radiation-induced
mutation and genetic engineering. They too became little more than tentacled brains that operate
fighting-machines. The cephalopod has certainly captivated many creative minds in the world! Martians, Illithids and Daleks happen to be some of my favourite monsters!
Is this a message from the future?
The fashionable illithid
Hanlon, R., Vecchione, M. and Allcock, L. 2018. Octopus, Squid & Cuttlefish. Ivy Press, UK.
Mather, J.A., Anderson, R.C. and Wood, J.B. 2010. Octopus, The Ocean's Intelligent Invertebrate: a natural history. Timber Press, Portland, Oregon.
Article last updated: 10/5/14
The squid body can be divided into three sections - the arm/tentacle crown, the head and the mantle. The mantle (or pallium) is like a 'cloak' that covers the body of the squid behind the head. It is an extension of the posterior body wall that extends forward, in a cone, to the head. Between the mantle and the body proper (visceral mass) is the mantle cavity. When the mantle expands, water is sucked into the cavity behind the squid's head. The mantle can then seal itself by means of valves and the squid can expel a jet of water through the ventral movable siphon (funnel), propelling it backwards at speed. This is important in escape responses when the squid evades a potential predator. The mantle will also pulse periodically to flush the gills with fresh oxygenated water. A pair of gills extend as outgrowths of the body wall that project into the mantle cavity. They are attached to the mantle by supporting suspensory membranes.
shell of Nautilus is divided into
chambers, with the oldest and smallest chamber at the apex of
the spiral and the body of Nautilus only occupies the most
recent and largest chamber. The chambers are separated by walls
of shell called septa (sing. septum). A tube of tissue called
the siphuncle traverses the length of the animal, passing
through each dividing wall (septum) in each shell chamber. The
siphuncle takes up salts from a water-filled chamber and water
follows by osmosis and then gas (mostly nitrogen) replaces the
water. This filling of a chamber with gas occurs whenever the
animal grows by secreting a new chamber into which the animal
moves, leaving behind a water-filled chamber in place of its
previous living compartment. Once the newly secreted septum is
strong enough to withstand the pressures, the vacated chamber is
filled with gas - counteracting the increase in mass of the
growing animal and keeping it buoyant. Extinct nautiloids
probably maintained buoyancy in a similar way. Cuttlefish also
maintain a shell comprising a mixture of fluid-filled and
gas-filled chambers, but this shell is the internal cuttlebone.
Cuttlefish can regulate the fluid/gas ratio in their
cuttlebones. During the day they lie buried in the sea bottom
and emerge at night to hunt for food. Light regulates this, with
buoyancy of the cuttlefish decreasing on exposure to light. Spirula is a cuttlefish whose
internal shell is coiled in a spiral, unlike typical cuttlebones
which are more-or-less straight shield-shaped structures.
Some squid, like Grimalditeuthis below, have secondary fins - an extra tail fin behind the main locomotory fin. The secondary fin lacks muscle and is thinner and more sheet-like and functions primarily as a flotation device.