Class Cephalopdoda overview

These are considered to be the most sophisticated molluscs, and possibly the most intelligent invertebrates. There are about 660 species and a large size range, the Giant squid (Architeuthis sp.), can be over 20 m long.

The cephalopod fossil record goes back to the Cambrian, and were similar to the nautilus (see below) extant today. They have a wide range of behaviors, a relatively large and complicated brain, and a great capacity for learning. All are predators.

Cephalopod body

The foot has developed into a number of prehensile arms with suckers around the mouth, with one or two modified for sperm transfer (see mating below); and a muscular funnel in the mantle cavity. This funnel is used in movement when water is forcibly expelled through it, a form of jet propulsion.

They have a radula and a pair of beak-like jaws. They also have an ink gland which releases a cloud of ink through the anus when the animal is alarmed. The ink cloud can act as a decoy to a predator allowing the cephalopod to escape.

There are two Sub-classes, the Nautiloidea with six species, and Coleoidea which includes the squids, octopuses and cuttlefishes. All are marine.


There are six species of Nautiloidea; they have a snail-like shell which is divided by septa into gas-filled chambers, with only the last chamber being occupied by the animal, see Nautilus pompilus, right and below. A fully grown adult may have as many as 30 chambers. The empty chambers are buoyancy aids.

The septa are perforated in the middle, and through this a cord called the siphuncle passes (see the diagram below) and secretes gas into the empty chambers. The siphuncle is a cord of tissue running through the shell of the nautiloid connecting all the chambers with the body of the animal.

The mouth is surrounded by up to 90 tentacles or arms; all suckerless.

Nautilus pompilus, whole shell and half shell showing internal chambers

Nautilus interanl anatomy

The hood acts as a cover or operculum when the animal withdraws into the shell. However, when the hood is closed it cannot breathe, so this only occurs in an emergency. The eyes and nervous system are relatively simple compared to the Coleoidea (see below), and the eyes have no lens.

All species are carnivorous.

Nautilus pompilus (above) is found in the Indo-Pacific region near the sea bed in waters up to 500 m deep. During the night it rises nearer the surface to feed. It is about 20 cm long when fully grown, and takes around 15 to 20 years to reach sexual maturity.

It swims backwards at about the same speed as a person swimming leisurely.

Cepalopod eye cross section

Coleoidea (squid, cuttlefish and octopus)

The Coleoidea have no external shell. Cuttlefish have an internal calcareous shell, squids have a thin cartilaginous pen, see Loligo sp. the common squid below, and in octopods the shell is absent, see Eledone sp. bottom left.

Octopuses tend to be solitary, but squid are often found in shoals making them a commercially viable species for fishermen to catch. Squid and cuttlefish have 10 arms - 8 short and 2 long. Octopods have 8 arms; all of the same length.

Coleoidea (squid, cuttlefish and octopus) eyes

The eyes are large, sensitive and share many features with vertebrate eyes (see left), e.g. iris, cornea, lens focused by muscles and retina. The pupil is slit-shaped and the slit is aligned so that it is kept horizontal.

The vertebrate eye arises from the development of the brain, whereas the cephalopod eye arises from the development of the skin. The eyeball of the giant squid is nearly 50 cm across!

loligo, common squid


The cephalopod chromatophore cell is a small sac filled with pigment and surrounded by muscle cells. These muscle cells are capable of stretching the pigment cell out so that it displays the colour. When the muscles cells are relaxed the pigment is more-or-less invisible. This type of cell allows for rapid colour changes.


Squid have a pair of long, club-ended arms that can be shot out with both great speed and precision to capture prey. The Giant squid (Architeuthis) are the largest invertebrates with a body length that can exceed 4 m and arms of 20 m. However, they, like all squid, will die after reproducing just once. Squid are a very important prey of sperm whales.

Above is Loligo sp. the common squid, which grows to about 30 cm long.

On the right is Loligo opalescens showing the major internal organs.

Loligo sp. feed on shrimp and fish, and cuttlefish feed mainly on shrimp and crabs.


Loligo opalescens, squid internal anatomy

Mating in the cephalopoda

Cephalopod males fertilise the female by inserting the hectocotylus (located at the end of one of their arms) into the female. In some cases the hectocotylus breaks off and stays in the female.

Before the mating habits were understood females found with a hectocotylus embedded in them were thought to have been parasitised.

Many species breed just once then die.

Vampire squid

On the right is the vampire squid, Vampyroteuthis infernalis. It lives in tropical and sub-tropical oceans in water between 700 - 3 500 m deep, and is purple/black in colour. It has light emitting organs (photophores) on its body, eyes and tentacles. Its arms are joined by a web of skin.

It is a very fast swimmer, and feeds on copepods, prawns and Cnidarians. Its Latin name translates as "vampire squid from hell"! More recently it has been compared to bankers. This, I feel, is unfair to the squid. It is true that both eat prawns, but only one has managed the seemingly difficult task of eating our future prawns, and getting us to pay for them as well. Prawns today, prawns tomorrow (apologies to Lewis Carroll). Any bankers who disagree can do so through the donation page.

Vampyroteuthis infernalis, vampire squid
Eledone sp. octopus

The blue-ringed octopus (Hapalochlaena lunata) is just 10 cm long. It is a very pretty animal and swims in an elegant manner among the coral reefs in the Pacific and Indian oceans. However it has a venomous bite potent enough to kill an adult human in just 15 minutes.

The giant Pacific octopus can be 5 m long and weigh up to 50 kg. The female lays her thousands of eggs in a sealed up burrow or crevice and looks after them washing them with a stream of water and grooming them to keep them free of parasites. The eggs can take eight months to hatch depending on the water temperature, and during all this time she does not feed. Soon after the eggs have hatched she dies. Although her size is large her life span is usually less than four years.

Left is Eledone sp., note that it has a single row of suckers, some octopuses have a double row.

Octopus by A C Hilton

Strange beauty, eight-limbed and eight-handed,
Whence camest to dazzle our eyes?
With thy bosom bespangled and banded
With the hues of the seas and the skies;
Is thy home European or Asian,
O mystical monster marine?
Part molluscous and partly crustacean,
Betwixt and between.

Wast thou born to the sound of sea trumpets?
Hast thou eaten and drunk to excess
Of the sponges -- thy muffins and crumpets,
Of the seaweed -- thy mustard and cress?
Wast thou nurtured in caverns of coral,
Remote from reproof or restraint?
Art thou innocent, art thou immoral,
Sinburnian or Saint?

Lithe limbs, curling free, as a creeper
That creeps in a desolate place,
To enroll and envelop the sleeper
In a silent and stealthy embrace,
Cruel beak craning forward to bite us,
Our juices to drain and to drink,
Or to whelm us in waves of Cocytus,
Indelible ink!

O breast, that 'twere rapture to writhe on!
O arms 'twere delicious to feel
Clinging close with the crush of the Python,
When she maketh her murderous meal!
In thy eight-fold embraces enfolden,
Let our empty existence escape,
Give us death that is glorious and golden,
Crushed all out of shape!

Ah! thy red lips, lascivious and luscious,
With death in their amorous kiss,
Cling round us, and clasp us, and crush us,
With bitings of agonised bliss;
We are sick with the poison of pleasure,
Dispense us the potion of pain;
Ope thy mouth to its uttermost measure
And bite us again!

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