Anatomy of the Octopus

So, you want to learn about the anatomy of the octopus? You’ve come to the right place.

Octopus biology is fascinating, because they’re so different from what we’re familiar with. Scientists estimate our nearest common ancestor was a flatworm in the Pre-Cambrian period, 600 million years ago. The only characteristics we shared then would have been a nervous system, and maybe eyes!

So the octopus has developed an entirely unique way to deal with the challenges of survival, optimized for their underwater environment. To help you get familiar with these squish cephalopods, here’s a guide to the notable characteristics of the octopus anatomy.

Optimal Invertebrates

Octopuses are true invertebrates, meaning they do not have spines, or any internal skeleton for that matter! The only hard part they have is their beak, but we’ll get to that later. This is why octopuses are notoriously flexible, and can squeeze into any space wider than their heads.

Octopuses have heads with two eyes like we do, but that’s where the similarities end. Extending from their mouth area are eight flexible arms that they use for grasping and manipulating. On the other end of their head is a big balloon-shaped body part called a mantle, where all their internal organs are kept. The mantle also houses the octopus respiratory system. Octopuses inhale water through a cavity at the base of the mantle. This water passes through the octopus’s gills, and back out through a flexible funnel called the siphon. Octopuses can direct the force of water that comes out of their siphon for jet propulsion to move quickly in any direction.

Octopus skin can change color and texture. Their skin is covered with color-changing organs and texture-changing papillae that can stiffen and smooth. Octopuses use these organs primarily for camouflage, but they are also sometimes involved in interspecies communication.

A Call to Arms

Octopuses have eight arms, and no tentacles. That’s right! Tentacles are specialized, stretchy limbs present only in other cephalopods (you can read more about that here). Though octopus arms appear extremely dynamic, they’re not elastic in the same way tentacles are. They stretch through muscular motion, just like your arms do!

Of course, they don’t have bones, so their stretching goes a bit further than a human’s. You may wonder: without bones, how to they create leverage against anything? Why don’t they just flatten out into a pancake, like a big bag of water? Octopuses like us are mostly water, but they take advantage of the incompressibility of water to move their arms. Their muscles stretch and compress against a system of muscular ‘hydrostats’ to stiffen and twist in incredibly dynamic ways.

World-wide Web

Arms are connected to each other by a thin film of skin called the web, which is used for enveloping prey. This web can be stretched out nearly to the tips of their arms, like an umbrella, or pulled close to allow for an individual arm to extend more easily.

Suction Section

You probably know that octopuses arms each have many, many cylindrical organs called suckers. These are how octopuses hold on to things!

Octopus suckers usually come in two parallel rows, extending all along the arm. They can control each sucker individually, so octopus arms are some of the most flexible limbs evolution has ever managed to develop.

Each sucker has a bell-shaped space in the center, just like a suction cup. When an octopus presses the outer part (infundibulum cavity) of its suckers against a surface, it can use muscles in its arm to increase the volume of the inner part (acetabulum chamber) and create a vacuum seal.

Arm Specialization

Most octopuses are the same all the way around (though there are some notable exceptions — for example the blanket octopus has four arms that are much longer than the rest of their body. But half of all octopuses have one arm that’s very specialized. It’s called the Hectocotylus.

Octopus hectocotyluses can vary widely by species, so this illustration is just one example of what the different parts might look like.

The Hecototylus is usually the third right (but sometimes the third left) arm of a male octopus, counting from the front two arms if you’re facing the octopus head on. It’s notable as the part of the body designed for reproduction. Male octopuses produce their reproductive material in sperm ‘packets’ called spermatophores, which they then need to deliver to the female’s mantle. To do that they will move a spermatophore along a specialized groove that runs all along the hectocotylus arm. Eventually it reaches the end of the arm, to a sucker-less section called the ligula. It’s secured in a triangular pocket called a ‘calamus’. The male octopus extends this part of their arm into the female octopus’s mantle to deliver their genetic material.

Read more on octopus arms and suckers here.

The Eyes have it

Despite evolving entirely separately from our own, octopuses developed camera eyes like ours. They have a pupil, surrounded by a flexible iris, and protected by a transparent cornea. There are some key differences, however. Their pupil has a horizontal shape to it, and their eyes don’t sit in a socket. Instead they have interocular muscles in a ring around the eye that move the whole organ along the head! Remember, octopuses are incredibly flexible. This means each eye can move independently from the other. Octopus eyes do not have eyelids, but these flexible ring muscles can move to cover the cornea in the same way our eyelids protect our eyes.

Read more on octopus eyes here.

Inside the Octopus

An octopus mantle has all the organs your expect for a complex organism, and them some! Let’s take a tour of the internal organisms of the octopus, starting with the last place that a tasty crab wants to be— the octopus’s mouth!

The Beak

It’s rare to see, but at the center of the octopus’s arms is the only hard of the octopus, a hard, parrot-like beak made of keratin. This sharp pointed instrument is used to break the shells of crustaceans and molluscs the octopus wants to eat. It sits in a muscular structure called the 'buccal mass’ that keeps it attached to the esophagus.

The octopus has a tongue-like appratus in their beak called a radula, which is kind of like a treadmill covered in sandpaper. They use this to drill holes into shells that are too hard to break apart.

Most octopuses have at least one venom gland along their esophagus. They deliver venom through their saliva. Often this serves as a paralytic agent to stop their prey from struggling.

Read more on octopus beaks here.

Brain(s)?

Octopus brains are unique, so much so that there are several articles diving into deeper detail about their incredible distributed nervous system that gives each arm semi-autonomy to hunt and solve problems.

What is quite unexpected is that the octopus central brain is shaped like a donut! It’s located in a ring that goes all the way around the esophagus! So if you’re an octopus who doesn’t chew your food, your’e at literal risk of getting brain damage. Octopuses have actually been found with this kind of injury!

Adjacent to the central brain are two optic lobes (conveniently next to the eyes) for visual processing. Extending arm-ward from the central brain are axial nerve cords that extend down each arm. Additionally, these nerve cords connect to each other in a nerve ring that allows for signaling to pass directly from arm to arm, bypassing the central brain entirely!

Read more on octopus brains here.

Digestive Tract

If you’re a thoroughly disassembled crab heading down to be digested by an octopus, you have to wait your turn. Like many vertebrate animals like birds and herbivores, octopuses have a crop, a prep station for food that isn’t ready to be digested yet. Fluids produced in the digestive gland break the proteins down into materials the octopus body can use. Once there’s room, food is transferred to the stomach, where the food is digested. Octopuses do not have a small intestine, the stomach absorbs the usable nutrients right there. Anything that can’t be digested gets shuffled on to the caecum (analogous to our large intestine) and out through the siphon, along with the octopus’s most recent exhalation.

Read more on octopus digestion here.

Respiratory & Circulatory Systems

We alluded to this earlier, but octopuses get the oxygen necessary to power their muscles through water inhaled through the cavity. Within the cavity are a pair of gills, organs that draw oxygen out of water and bonded to special proteins in the octopus’s blood called hemocyanin. Each gill has its own gill heart which, with the help of a central systemic heart, pump the newly oxygenated blood throughout the octopus’s body.

The deoxygenated water is meanwhile exhaled through the siphon.

Read more on octopus circulation here and respiration here.

What’s a Siphon a’for?

It sure seems like siphons are sure used for a lot! The respiratory system and the digestive system both use it to get rid of the processed components they have no need for. And the resourceful octopus has managed to turn this system exit point as a means of locomotion, and they compress their whole body to exhale water quickly and move in a form of jet propulsion.

Another important use of the siphon is for defense. Along the digestive system is an ink sac, an organ that produces mucus and melanin, the opaque brown-black pigment that gives ink its color (and the same sun-protecting pigment found in human skin). If an octopus wants to distract or confuse a potential predator, they will add a burst of ink with their jet of water. This ink billows out behind the octopus, interfering with both the predator’s visual field and also their senses of smell and taste. You might think ink would smell bad, like a skunk, but it’s actually the opposite. Ink tastes pretty good, even though it’s not nutritious it’s an easier snack than an octopus. You yourself may have tried some cuttlefish or octopus ink in certain pasta dishes!

Read more on octopus ink here.

Whole body work-out

And with that we’ve explored the anatomy of the octopus from ceph to pod (head to foot)! If you’re looking for a deeper dive into any specific body parts, check out the links above!

If you’d like to commemorate this tour of the octopus body, you can get your very own octopus anatomy poster that collects my illustrations.

And you are?

Oh yeah, I’m SS Julian, author of the comic Octopolis! My comic is a science-fiction about a future society of intelligent octopuses. Though fiction, I wanted Octopolis to be as authentic and educational as possible, so I learned as much as I could about the unique biology and behavior of the octopus in order to write and illustrate the story. If you’re a fan of octopuses (and if you’re here on Octonation, you must be), you’ll probably enjoy Octopolis!

I also illustrated this short comic about octopus evolution written by Danna Staaf: How the Octopus Lost Its Shell.