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Home › Sea Life › Marine Animals › Invertebrate › Mollusks › Octopus › Anatomy

Interesting Facts about Octopus Body Parts

If you're describing a marine species that has a large bulbous head with four pairs of arms that look like tentacles, it has to be a member of the class Cephalopoda.

The information in this segment explains the physiological and physical characteristics of octopuses, including how their anatomical limbs, organs, and appendages work.

What is the Basic Anatomy of the Octopus?

Imagine an animal with neither an internal or an external skeleton. Yet, it is one of the most intelligent of all marine invertebrates.

Despite this fact, or because of it, they can squeeze through very small gaps and spaces that measure around 10% of their body size.

So, there are about 300 octopus species, which accounts for 30% of all known cephalopods.

You can find octopuses in every ocean around the world, particularly in shallow marine habitats and intertidal regions of the Pacific and Indian Oceans.

The two major groups of octopus species are the "finned" type (known as Cirrata) and those without "fins", called Incirrata.

Cirrate octopi have a pair of ear-like fins attached to the mantle (head) and tiny projections (called "Cirri") on their arms. A good example of the Cirrina suborder is the Dumbo octopus (scientific name "Grimpoteuthis").

Composition of an Octopus

Mantle Cavity

An octopus has a large muscled structure that contains its gills and internal organs. The visceral hump (known as a mantle) creates a strong muscular wall that assists with respiration, contraction, and it creates some protection for the body parts inside.

Internal Shell (Skull)

Most of the octopus phylum do not have any internal shells. However, Cirrate octopuses have a stiff well-developed calcium carbonate shell structure secreted by the mantle. Also, some species have a bony structure (cartilage) that encloses and protects the brain.

Brain and Buccal Mass

Like most animals, the octopus's doughnut-shaped brain is the vital organ that controls the nervous system.

The buccal mass is a muscular bulb containing:

• Mouth (underneath the arms)
• Chitinous beak
• Pharynx
• Radula (muscular tongue used to cut and scrape food)
• Salivary glands

Fun Fact: Octopus and cuttlefish have the highest brain-to-body mass ratio of all sea invertebrates. The common octopus (Octopus vulgaris) has around half a billion neurons (almost the same as a dog).

6 Arms, 2 Legs, 0 Tentacles

Octopus have four pairs of arms that they use to capture and hold their prey, such as crustacean animals, and for propulsion in the water. Being nocturnal carnivores means they also eat crabs, shrimps, and lobsters.

Yet, contrary to common belief, they do not have typical tentacles like most other mollusks (e.g. some species of sea snail).

But an octopus's mobile muscular appendages are so similar in function and structural characteristics that they're often termed as tentacles.

Here's the thing:

Scientific studies suggest they are bipedal, meaning two-footed. Unlike most cephalopod molluscs, octopus body parts are almost entirely soft. The only rigid part in octopus anatomy is the parrot-like beak.

Consequently, they can squeeze into tiny holes and crevices to escape their main predators, which are moray eels, sharks, and some marine mammals (e.g. dolphins).

Interesting Fact: Octopus arms can grow back after separation from its body. If they lose a limb, they make an escape arm-less and leave the damaged appendage wriggling and crawling around the sea bed - sometimes for several hours.

Hepatopancreas (Midgut Gland)

In aquatic mollusks, the digestive gland is the organ that produces a secretion to aid digestion. Other functions of the hepatopancreas include lipid metabolism, the storage of lipids, and the degradation of lipid cells (especially in gastropod molluscs).

Gastrointestinal Tract (Crop)

The passageway of the digestive system contains a large, thin-walled sac called a crop. This part of the alimentary tract is where octopuses store their food before digestion takes place inside the stomach.

Gills and Funnel

A funnel (also called a siphon) connects the muscular walls of the mantle cavity to the surrounding water. The two gills and branchial hearts extract oxygen from the water as part of the circulatory process.

Heart and Blood

Octopuses have three (3) hearts, a systemic heart and two gill hearts. Similar to the human heart, these muscular organs circulate blood around the animal's body.

The copper-rich protein called "haemocyanin" is the reason for the blue colour of octopus blood. But, this viscous substance requires a lot of pressure to pump it around the body. Hence, octopuses get tired when they swim long distances.

Nephridia (Kidneys)

Most marine invertebrates have a pair of kidneys (called nephridium) that perform the general function of those in humans (e.g. extract metabolic waste products from the blood).

Suckers

One of the distinguishing features of octopus anatomy is several hundred circular, adhesive suckers located on the inside of each arm.

Octopus use their muscular suckers to anchor themselves to objects or for manipulation. The Giant Pacific Octopus (Enteroctopus dofleini) can have up to 2,240 suckers.

Cephalopod Ink Sac

The ink sac is a muscular bag found in some cephalopod mollusks. Octopus use their sac to squirt a cloud of dark coloured ink (melanin) to try and confuse their natural predators.

Male Octopus Gonad

Being gonochoristic creatures means males and females have different genital organs. The single male gonad and female ovary (e.g. sperm or eggs) are important body parts for octopus reproduction - even though both parents will die shortly after copulation.

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