Octopus Anatomy: Labeled Diagrams and Interactive 3D Model

An octopus is a cephalopod mollusc with three body regions: the mantle, a muscular sac that holds the organs; the head, which carries the eyes and brain; and eight arms that surround the mouth and beak. It has no bones. It has three hearts, blue copper-based blood, and about 500 million neurons, two-thirds of them in its arms.

By convention, the arms are the front (anterior) of the body, the tip of the mantle is the back (posterior), and the funnel side is the underside (ventral). The arms are numbered L1–L4 and R1–R4 from the dorsal side to the ventral side.

The outside of an octopus shows the mantle, head, two eyes, the funnel (siphon), the mantle opening, eight arms lined with suckers, and the web of skin between the arm bases. The rounded “head” you see is actually the mantle; the true head sits between the mantle and the arms.

Males can be told apart by the hectocotylus: the tip of the third right arm (R3) has no suckers and ends in a spoon-shaped ligula. Each eye has a horizontal slit pupil, and a small olfactory pit behind and below each eye detects smells carried in the water.

Inside the mantle, the visceral mass holds the digestive gland, stomach, caecum, ink sac, three hearts, kidneys and gonad, while the ventral mantle cavity holds the two gills. The brain sits in the head inside a cartilaginous cranium, wrapped around the esophagus.

This is the text version of the 3D model: every structure, grouped by body system, with its location and function.

An octopus has one central brain, not nine. The brain is a ring of lobes around the esophagus, flanked by two large optic lobes. The “nine brains” idea counts the axial nerve cord of each of the eight arms as a mini-brain, because about two-thirds of the octopus’s roughly 500 million neurons are in the arms and each arm can control many movements on its own.

• Supraesophageal mass: Higher control: learning, memory and decision-making. The vertical lobe and frontal lobes are the main learning and memory centres.
• Subesophageal mass: Lower motor centres: the brachial lobe (arms), the pedal lobe (funnel and eye muscles), the palliovisceral lobe (mantle, breathing, ink release and the hearts) and the chromatophore lobes (skin colour).
• Optic lobes: Process visual input, store visual memories and help control body patterning for camouflage.
• Optic glands: Act much like the vertebrate pituitary gland. Their secretions trigger maturation of the gonads.
• Axial nerve cords: Coordinate arm and sucker movement locally. A ganglion for each sucker lets the arm carry out reflexes with little input from the brain.
• Stellate ganglia: Receive the pallial nerves from the brain and send stellar nerves out to the mantle muscles that drive breathing and jet propulsion.

Each arm cord has a ganglion for every sucker and is linked to the other cords by a nerve ring near the arm bases. The brain sends broad commands (“reach there”); the arm works out much of the detail itself. This is why a severed arm can still reach and grasp for a while.

An octopus has three hearts. Two branchial hearts, one at the base of each gill, pump blood through the gills. The systemic heart receives the oxygenated blood through two auricles and pumps it around the body through the aortas.

The blood is blue because it carries oxygen on haemocyanin, a copper-based protein dissolved in the plasma, rather than on iron-based haemoglobin in red cells. Haemocyanin carries less oxygen than haemoglobin, and the extra branchial hearts help push blood through the gills. The circulation is closed: blood stays inside vessels. When an octopus jets, pressure in the mantle stops blood returning to the systemic heart, which briefly stops beating (Wells et al., 1987).

An octopus breathes with two feather-like gills (ctenidia) inside the mantle cavity. The mantle muscles pump water in through the mantle opening, across the gills, and out through the funnel.

• Each gill is made of folded lamellae that give a large surface for gas exchange.
• Blood flows through the gill in the opposite direction to the water (countercurrent exchange).
• The gills have no cilia; all water movement comes from the mantle muscles.
• A branchial heart at the base of each gill pumps deoxygenated blood through it.

Food passes from the beak and buccal mass, down the esophagus (which runs through the brain), into the crop for storage, then to the stomach, caecum and digestive gland, and finally out through the intestine and anus into the mantle cavity, where exhaled water flushes it out of the funnel.

1. Beak: two chitinous jaws that bite prey into pieces small enough to pass through the brain.
2. Buccal mass and radula: the radula, a ribbon of tiny teeth, pulls food back into the esophagus. The toothed salivary papilla can drill through shells.
3. Salivary glands: the small anterior pair make mucus; the large posterior salivary glands make venom and enzymes that paralyse and start digesting prey.
4. Esophagus and crop: the crop stores food so the octopus can eat quickly and digest later.
5. Stomach and caecum: the stomach mixes food with enzymes; the spiral caecum sorts particles and absorbs nutrients.
6. Digestive gland: the largest organ, often called the “liver”. It makes enzymes and absorbs and stores nutrients.
7. Intestine, ink sac and anus: the ink duct joins the rectum, so ink and faeces leave by the same route.

Both sexes have a single gonad at the back of the visceral mass. Males have a testis, a spermatophoric complex that packs sperm into spermatophores, and a hectocotylus arm to hand them over. Females have an ovary and paired oviducts with oviducal glands that store sperm.

Sexual maturation is switched on by the optic glands. In brooding females, removing these glands made them stop brooding, resume feeding and live for months longer (Wodinsky, 1977).

An octopus swims by jet propulsion: it fills its mantle cavity with water, then contracts the mantle to force the water out of the funnel, which pushes the body the opposite way, usually mantle-first.

Radial muscles in the mantle wall expand the cavity; circular muscles squeeze it. The funnel can be pointed to steer. The same pumping, done gently, is how the octopus breathes. Jetting is fast but expensive, so octopuses usually crawl.

The common octopus lives about one to two years and breeds only once. After mating, the female lays strings of eggs in a den and guards them until they hatch into tiny planktonic paralarvae, which later settle on the sea floor as juveniles. The female dies around the time her eggs hatch.

All octopuses share the same basic anatomy shown on this page, but some species differ in striking ways. Dumbo octopuses belong to a separate group, the cirrate octopods, with fins and finger-like cirri on their arms.

Blue-ringed octopus anatomy

Blue-ringed octopuses carry tetrodotoxin (TTX), a nerve poison that can kill a human, in their posterior salivary glands and other tissues. Where the TTX comes from is still debated: bacteria able to make TTX have been found in the salivary glands, but this has not been proven to be the source (Whitelaw et al., 2020). The blue rings are iridophores (light-reflecting cells) that are hidden or exposed by muscles, so the rings can flash in under a second (Mäthger et al., 2012).

Dumbo octopus anatomy

Dumbo octopuses are cirrate octopods. Two ear-like fins on the mantle drive swimming, supported by a U-shaped internal shell. Their arms carry a single row of suckers alternating with finger-like cirri, and a deep web joins the arms. They lack an ink sac and, unlike the common octopus, cannot change colour. One was filmed at 6,957 m in the Java Trench, one of the deepest cephalopod sightings (Jamieson & Vecchione, 2020).

Giant Pacific octopus anatomy

The giant Pacific octopus is the largest octopus species. Its anatomy is the same as the common octopus, on a larger scale. It lives about three to five years, longer than most octopuses, and the female broods her eggs for about six months without eating before she dies.