According to Merriam-Webster , brains are the central processing units (organs) whose function is to control functionalities of a living body. Who needs—or could even use—more than one brain? Certainly not humans!

But consider other animals, living other lives, needing to function differently to live and reproduce.

The current (10-09-2025) issue of The New York Review of Books had a l-o-n-g article on five (!) books about octopuses (Such Flexible Intensity of Life by Verlyn Klinkenborg). The first paragraph includes the sentence, “An octopuses’s brain is distributed throughout its soft, amorphous body: nine brainlike (sic) nerve clusters, one at the base of each arm and another at its head.” It’s a fun read about how octopuses live, why each arm needs a brain, and what several writers have to say about them.

My immediate thought was “Do other animals have more than one brain?” Read on to learn what I discovered!

Multiple Brains

For many animals, one brain simply isn’t enough for the job.

Cephalopod Brain Arrangements

Cephalopods include octopuses, squids, nautiluses, and cuttlefish.

These animals all possess many arms. Their central brains form a ring around their esophagus and their arms are constantly testing the environment, processing the information they gather, “thinking” for themselves.

Octopuses have nine brains. The main one is in their head. In addition, each arm can touch, control, and move independently of the others thanks to the cluster of neurons in it. This is part of what makes them so smart. They’re able to figure out puzzles, open containers, and untie knots. In total, octopuses have more than 500 million neurons, only 30% located in the central brain. The brains don’t have to communicate with one another to work, but they can stay in touch to be more effective. (For more fascinating details, check out Ze Facts’ video True Facts About The Octopus.)

Squids have three brains. The main one is used to control the body as a whole while the other two (optic lobes) are dedicated to the eyes. Squid brains resemble donuts in shape. Though there are three brains, they are relatively small compared to the size of the body. Based on research on squids, approximately 80 percent of the brainpower goes toward processing visual information.

Cuttlefish are cousins to octopuses and squids. With 500 million neurons dispersed throughout the body, some say they have 9 brains. Cuttlefish can count up to 5, on par with infant humans and young monkeys. They can delay gratification: hungry cuttlefish will resist mediocre treats for tastier ones delivered later, a sign of intelligence thought to be important for decision making and planning. When cuttlefish were trained to remember when and what they were fed, older cuttlefish did just as well as the young cuttlefish even though they showed other signs of aging

Insect Brain Arrangements

Scientists estimate there are more than 900,000 different types of insects. I’m concerned here with the brain arrangements of cockroaches, bumblebees, and mosquitoes.

Bumblebees have three brains as well. One controls the wings, another controls the stinger, and the third manages the antennae. These brains are more focused on instinct and basic memorization than actual memories and behaviors. Compared to other insects and their sizes, bumblebee brains are rather small. However, they are able to remember flowers and areas full of pollen for future use. They can find their way to previous locations, new areas, and back home.

Mosquitoes have three brains inside their body. The first is in the head, the second in the wings, and the third in the abdomen. These brains tend to focus on working the nearest body part. Mosquitoes have a central brain that runs the whole body, one that runs the wings, and one that runs the legs and stomach. Though their brains are tiny, about 1/1000th the size, and only 100 million neurons compared to 10 billion in people, they are still rather smart. They learn to avoid certain predator habitats, and how to find food and water via smell.

Cockroaches have two brains, one in the head, the other near the abdomen. This is what allows cockroaches to continue to move for a while even if they become decapitated. Though they might live for a minute, they will still die shortly after decapitation. The two brains also allow cockroaches to move fast. They are also intelligent, remembering where they left items and recognizing various objects. They can even remember faces. Though their brains are about 1/20th the size of a human’s and have about a third of the number of neurons, they have twice as many synapses. These synapses are what pass information back and forth. This allows for memory storage and keeps the brain active.

Annelid Brain Arrangements

Annelids, aka segmented worms, include more than 20,000 species, none of which you’d want to invite for dinner. Many of them, including leeches and silkworm moths, have very interesting brain configurations.

Leeches may have the most brains of any animal. A leech’s body contains 32 different segments. Each of these segments has its own brain. These brains are actually small ganglions and are responsible for their specific section. Each of these 32 different brains have about 400 neurons. Together, there are about 500 unique types of neurons in one leech. This design might help them make faster decisions, slow down the aging process, or allow them to use brains in place of nerves.

Silkworms and silkworm moths, also have quite a few brains. They have 11 distinct brain lobes; other moths and butterflies don’t have as many lobes. Additionally, most other similar insects form one brain made up of several lobes, rather than distinct lobes. The brains of silkworms are less than a tenth of the size of a human brain. However, they’ve been shown to remember and learn information rather than just relying on instincts.

Gastropod Brain Arrangements

Snails and slugs, the two types of gastropods, have very similar brain structures. However, they’re not entirely the same.

Snails have three to six brains! Originally, species had two sets of ganglia, or brains, for movement, breathing, and eating. However, in many species, the two sets fused into one and formed a nerve ring around it. In some species, especially the more primitive ones, it’s more likely to see two sets still, for a total of six brains instead of three.

Despite being fragile and simple, slugs have three brains. They are located in the esophagus and are called buccal, cerebral, and pleural. The buccal is used for feeding, the cerebral is used for movement and senses, and the pleural manages breathing and respiration. For a long time, slugs were thought to have pretty weak brains overall, especially considered to be weaker than average mammal brains. However, scientists found that slug brains work similarly to humans in some ways. They save memories and patterns in the same way the average human would.

Arachnid Brain Arrangements

Arachnids, which are not insects, by the way, have only one brain. Nevertheless, they can behave in ways that seem extra intelligent. Here, I’ll deal only with spiders as an example.

Spiders have large brains for their size, some spiders having brains that take up 80% of the body. The brain of a spider sits inside its fused head and thorax, surrounded by muscle and other tissues. The large brains are important for functions like web building or hunting. Behaviors that can be described as “cognitive,” as opposed to automatic responses, are fairly common among spiders. From orb weavers that adjust the way they build their webs based on the type of prey they are catching to ghost spiders that can learn to associate a reward with the smell of vanilla , there’s more going on in spider brains than they commonly get credit for.

Single Brains

Mammals normally have only one brain. However, exceptions can occur in dogs, monkeys, cats, and even human beings — but these are mainly due to polycephaly, a rare genetic anomaly when an animal is born with two heads. Which means two brains.

No Brains

And there are some animals with no brains or nervous tissue of any kind.

They tend to be extremely simple animals possessing rudimentary biological characteristics sufficient for survival. Many are immobile, using sensory nerves to “find” food and avoid danger. Oddly enough, they are similar to technology (such as computers and smartphones), which also use sensors to detect light, movement, chemicals, and so on. Brainless animals operate in the same manner, allowing them to do the things they need to survive.

A Few Examples

Sponges survive on the sea floor by taking nutrients into their porous bodies. There are conflicting theories as to whether sponges have always been this way or evolved to get rid of their brains to be more energy efficient.

Sea urchins are pointy, spiky animals, you don’t want to step on while at the beach. Fortunately, outside of South Florida, sea urchins are not poisonous. They have various numbers of legs and control feeding by way of a water vascular system. That system changes the amount of pressure and water in its body, in order to move faster. (Starfish operate the same way.) Its mouth is beneath it. They expel waste from the top of their bodies. The sea urchin sits on stones, scraping and feeding on the algae that helps keep the ocean clean.

Sea cucumbers look like worms and feed on plankton. They’re everywhere and extremely dangerous. They can release a toxic substance (holothurin), which can blind humans permanently. However, without a brain, they’re not a deliberate threat. There are more than a thousand types of sea cucumbers. Many of the species live in deep water. They feed instinctively, using tubular feet around the mouth to catch and take aquatic invertebrates, algae, and waste. These creatures exhibit asexual and sexual reproduction.

Jellyfish (or “jellies”) are a family of translucent animals, famous—perhaps infamous—for their sting. When their tentacles touch a foreign object (such as a swimmer) they release a shooting sting. That sting releases a toxin capable of neutralizing or killing most intruders, though humans usually experience only temporary misery. Jellyfish are uniquely mobile. All other brainless animals are pretty much stationary, often spending their entire lives in a single spot. However, jellyfish move with the current in oceans and bays. They also squirt water to move forward. Thus they can propel themselves toward prey, such as plankton. Their ability to move also helps them avoid predators, such as fish, turtles, and seabirds. The jellyfish functions through a network of sensory nerves.

Bottom Line: Animals have the number, size, and structure of the brains they need to live, function, and reproduce.

Additional Fun Fact: Last week, I talked about how there are museums for seemingly everything out there. I’ve found a Museum of Scientifically Accurate Fabric Brain Art at the University of Oregon!