13 Things You Didn't Know About Planarians

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Planarians—free-living flatworms found around the world in terrestrial, marine, and freshwater environments—are fascinating organisms with an interesting natural history ... and a rather complicated relationship with biomedical research. Many species of these worms can regrow their body parts, including a new head, upon decapitation, and are therefore one of the organisms of choice in regeneration research. But they're also being “rediscovered” in several other research areas, including pharmacology and the neurosciences. And they're also pretty cute. Here are some things you probably didn’t know about them.

1. Some species practice chemical warfare.

There are several marine planarian species and at least one terrestrial planarian species that produce tetrodotoxin, one of the most lethal substances known. There is no known antidote.

2. Their mouth is located at the middle of their body—and it’s not only used for eating.

A.G. Pagán. Adapted from The First Brain: The Neuroscience of Planarians. Copyright Oxford University Press.

All planarians are meat-eaters (cannibals, even); most are active predators or, at the very least, scavengers. When hunting small prey, like water fleas, planarians wrap themselves around their prey just like a constrictor snake. Mealtime plays out like a scene from a horror movie: To eat, the worms extend a tube-like organ, called a proboscis or pharynx, which is located at the center of their body. Their mouth is located at the pharynx’s tip, which also acts as an anus.

3. They almost were the animal model that defined genetics in the 20th century.

In the early 20th century, Thomas Hunt Morgan —the father of modern genetics— considered planarians and the fruit fly (Drosophila melanogaster) as potential animal models. He chose Drosophila, and the rest is scientific history.

4. Most planarian species have two eyes, which always look “crossed”; nobody knows why.

O.R. Pagán.

There are also species with many eyes, distributed throughout their bodies; species with only one eye; and eyeless species. Weirdly, a decapitated planarian body can detect light, and many planarian species have ear-like structures on their heads that don't detect sound but chemicals; you could say that they taste and smell with their ears.

5. There was an actual comic book titled Planarian Man.

Neal Obermeyer

Planarian Man is the creation of Neal Obermeyer, a journalist and editorial cartoonist from Omaha, Nebraska. As expected, Planarian Man is a superhero who is half planarian and half human. His origin story states that he came to be when a young boy cut his finger while slicing a planarian as part of a high school experiment. A small piece of the worm got into the cut and eventually the boy began changing, until he became the crime-fighting Planarian Man.

6. Planarians display behaviors very similar to addiction when given many of the same drugs that humans abuse.

For quite a long time, scientists gave planarians a wide variety of drugs and other chemicals to explore aspects of physiology, but not to study specific areas like addiction. That changed in 2001, when a group led by Dr. Robert Raffa of Philadelphia's Temple University published a paper describing planarian behaviors resembling “withdrawal symptoms” upon exposure to cocaine. This paper triggered a renewed interest on the systematic research on planarian pharmacology. Since then, many other drugs have joined the list of substances that can induce addiction-like behaviors in these worms, including nicotine, amphetamines, and cannabinoids, among many other drugs. Now, speaking of pot…

7. Planarians Have been mentioned on The Big Bang Theory.

One of Theory's main characters is neuroscientist Amy Farrah Fowler, who is played by actual neuroscientist Mayim Bialik. In the episode "The Monster Isolation," which first aired on February 21, 2013, Amy was complaining about some nicotine-addicted monkeys that were going through withdrawal in a rather rough way, then said, “... This makes me miss my marijuana-abusing flatworms; those guys were mellow!”

8. Two other TV shows also mentioned planarians... Both in the same day and the same time!

On March 16, 2014, both The Walking Dead and the reimagined Cosmos documentary series, hosted by Neil deGrasse Tyson, talked about planarians. In The Walking Dead episode “The Grove”, a girl named Mika says, “I miss science class. Except when we had to do gross stuff like cut up planaria worms.” And in the episode “Some of the things that molecules do,” Cosmos used planarians in a segment about evolution. Unfortunately, the animation featured a freshwater planarian in a marine environment, and the animation moved nothing like a freshwater planarian. I guess I should be happy that they showed planarians at all.

9. Many species of planarians can regenerate lost parts—including their heads, which contain rudimentary but fully-functional brains.

A.G. Pagán. Adapted from The First Brain: The Neuroscience of Planarians. Copyright Oxford University Press.

If you cut a planarian into several pieces, over time each piece will regenerate into a complete worm. And if you decapitate a planarian, it will not die. The head will keep on living and moving, and will eventually regenerate a new body. The headless body eventually will regenerate a new head, including its brain and nervous system.

Speaking of nervous systems, in contrast to most bilateral animals (those that display unambiguously right and left sides), planarians have not one, but two nerve cords that run along their bodies. It's unclear why they need two of them.

10. If you cut planarians in a particular way, you can make them grow multiple heads.

Yes, you read that right: Like the mythical hydra, planarians can sprout several heads—in some cases, as many as 10. The phenomenon was first reported in 1814, in John Graham Dalyell's book, Observations of Some Interesting Phenomena in Animal Physiology, Exhibited by Several Species of Planariae. (Interestingly, Dalyell was not a trained scientist, but a lawyer; you can read more about the amateur naturalist in this book.) More recently, scientists have begun to unravel some of the molecular mechanisms that can trigger the development of multiple heads in planarians. This paper, though quite technical, has some really nice pictures of multi-headed worms.

And it gets better (or weirder!). If you cut a planarian in half, normally the head portion develops a new tail and the tail portion develops a new head. Very recently, scientists have learned how to make a tail portion grow another tail in place of a head and an anterior portion to develop another head instead of a tail. Here are a couple of examples:

Dr. Junji Morokuma, Levin lab, Tufts University

11. Planarians are capable of learning, and upon decapitation, the bodies with newly regenerated heads will remember what they learned.

In the 1950s and 1960s, experimental psychologist James V. McConnell and collaborators did a series of experiments using planarians to explore memory processes. Some of these experiments seemed to indicate that if you trained planarians to respond to certain stimuli, not only did they remember, but if you cut their heads off and allowed the bodies to regenerate a new head, many of the regenerated worms actually remembered their training!

For a series of complex reasons, a significant fraction of the scientific community did not trust these experiments, citing problems with the use of appropriate controls, observer bias, and other less polite reasons. But in 2013, a group led by Dr. Mike Levin at Tufts University published a really interesting paper where they conclusively demonstrated that planarians can indeed learn, and that the tail will remember.

12. If you transplant the brain of a planarian to the body of another planarian, the transplant will hold and eventually control its new body.

There are several examples of experiments where researchers removed a planarian’s brain and “installed” it in another worm’s body, with at least partial recovery of function. As if this were not remarkable enough, sometimes these brain transplants were successful even when the two worms belonged to different species. Even more bizarre experiments explored what would happen if they dissected a marine planarian brain and put it back in the same worm at an angle, sometimes even backwards. Again, the worms displayed at least some partial recovery. You can find a more detailed account of these experiments my book, The First Brain: The Neuroscience of Planarians.

13. DARWIN STUDIED PLANARIANS.

Charles Darwin himself studied certain land planarians and, although not the first to observe it, he noted their regenerative abilities. In his own words:

“Having cut one of them transversely into two nearly equal parts, in the course of a fortnight both had the shape of perfect animals. I had, however, so divided the body, that one of the halves contained both the inferior orifices, and the other, inconsequence, none. In the course of twenty-five days from the operation, the more perfect half could not have been distinguished from any other specimen.”

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Roadside Bear Statue in Wales is So Lifelike That Safety Officials Want It Removed
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Wooden bear statue.

There are no real bears in the British Isles for residents to worry about, but a statue of one in the small Welsh town of Llanwrtyd Wells has become a cause of concern. As The Telegraph reports, the statue is so convincing that it's scaring drivers, causing at least one motorist to crash her car. Now road safety officials are demanding it be removed.

The 10-foot wooden statue has been a fixture on the roadside for at least 15 years. It made headlines in May of 2018 when a woman driving her car saw the landmark and took it to be the real thing. She was so startled that she veered off the road and into a street sign.

After the incident, she complained about the bear to highways officials who agreed that it poses a safety threat and should be removed. But the small town isn't giving in to the Welsh government's demands so quickly.

The bear statue was originally erected on the site of a now-defunct wool mill. Even though the mill has since closed, locals still see the statue as an important landmark. Llanwrtyd Wells councilor Peter James called it an "iconic gateway of the town," according to The Telegraph.

Another town resident, who wished to remain anonymous, told The Telegraph that the woman who crashed her car had been a tourist from Canada where bears are common. Bear were hunted to extinction in Britain about 1000 years ago, so local drivers have no reason to look out for the real animals on the side of the road.

The statue remains in its old spot, but Welsh government officials plan to remove it themselves if the town doesn't cooperate. For now, temporary traffic lights have been set up around the site of the accident to prevent any similar incidents.

[h/t The Telegraph]

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10 Scientific Benefits of Being a Dog Owner
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The bickering between cat people and dog people is ongoing and vicious, but in the end, we're all better off for loving a pet. But if anyone tries to poo-poo your pooch, know that there are some scientific reasons that they're man's best friend.

1. YOU GET SICK LESS OFTEN.

Dog snuggling on a bed with its person.
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If cleaning commercials are to be believed, humanity is in the midst of a war against germs—and we shouldn't stop until every single one is dead. In reality, the amount of disinfecting we do is making us sicker; since our bodies are exposed to a less diverse mix of germs, our entire microbiome is messed up. Fortunately, dogs are covered in germs! Having a dog in the house means more diverse bacteria enters the home and gets inside the occupants (one study found "dog-related biodiversity" is especially high on pillowcases). In turn, people with dogs seem to get ill less frequently and less severely than people—especially children—with cats or no pets.

2. YOU'RE MORE RESISTANT TO ALLERGIES.

Child and mother playing with a dog on a bed.
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While dog dander can be a trigger for people with allergies, growing up in a house with a dog makes children less likely to develop allergies over the course of their lives. And the benefits can start during gestation; a 2017 study published in the journal Microbiome found that a bacterial exchange happened between women who lived with pets (largely dogs) during pregnancy and their children, regardless of type of birth or whether the child was breastfed, and even if the pet was not in the home after the birth of the child. Those children tested had two bacteria, Ruminococcus and Oscillospira, that reduce the risk of common allergies, asthma, and obesity, and they were less likely to develop eczema.

3. YOU'LL HAVE BETTER HEART HEALTH.

Woman doing yoga with her dog.
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Everything about owning a dog seems to lend itself to better heart health. Just the act of petting a dog lowers heart rate and blood pressure. A 2017 Chinese study found a link between dog ownership and reduced risk of coronary artery disease, while other studies show pet owners have slightly lower cholesterol and are more likely to survive a heart attack.

4. YOU GET MORE EXERCISE.

Person running in field with a dog.
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While other pets have positive effects on your health as well, dogs have the added benefit of needing to be walked and played with numerous times a day. This means that many dog owners are getting 30 minutes of exercise a day, lowering their risk of cardiovascular disease.

5. YOU'LL BE HAPPIER.

Woman cuddling her dog.
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Dog owners are less likely to suffer from depression than non-pet owners. Even for those people who are clinically depressed, having a pet to take care of can help them out of a depressive episode. Since taking care of a dog requires a routine and forces you to stay at least a little active, dog owners are more likely to interact with others and have an increased sense of well-being while tending to their pet. The interaction with and love received from a dog can also help people stay positive. Even the mere act of looking at your pet increases the amount of oxytocin, the "feel good" chemical, in the brain.

6. YOU HAVE A MORE ACTIVE SOCIAL LIFE.

Large bulldog licking a laughing man.
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Not only does dog ownership indirectly tell others that you're trustworthy, your trusty companion can help facilitate friendships and social networks. A 2015 study published in PLOS One found that dogs can be both the catalyst for sparking new relationships and also the means for keeping social networks thriving. One study even showed that those with dogs also had closer and more supportive relationships with the people in their lives.

7. YOUR DOG MIGHT BE A CANCER DETECTOR.

Man high-fiving his dog.
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Your dog could save your life one day: It seems that our canine friends have the ability to smell cancer in the human body. Stories abound of owners whose dogs kept sniffing or licking a mole or lump on their body so they got it checked out, discovering it was cancerous. The anecdotal evidence has been backed up by scientific studies, and some dogs are now trained to detect cancer.

8. YOU'LL BE LESS STRESSED AT WORK.

Woman working on a computer while petting a dog.
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The benefits of bringing a dog to work are so increasingly obvious that more companies are catching on. Studies show that people who interact with a pet while working have lower stress levels throughout the day, while people who do not bring a pet see their stress levels increase over time. Dogs in the office also lead to people taking more breaks, to play with or walk the dog, which makes them more energized when they return to work. This, in turn, has been shown to lead to much greater productivity and job satisfaction.

9. YOU CAN FIND OUT MORE ABOUT YOUR PERSONALITY.

Man running in surf with dog.
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The kind of dog you have says a lot about your personality. A study in England found a very clear correlation between people's personalities and what type of dogs they owned; for example, people who owned toy dogs tended to be more intelligent, while owners of utility dogs like Dalmatians and bulldogs were the most conscientious. Other studies have found that dog owners in general are more outgoing and friendly than cat owners.

10. YOUR KIDS WILL BE MORE EMPATHETIC.

A young boy having fun with his dog.
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Though one 2003 study found that there was no link between pet ownership and empathy in a group of children, a 2017 study of 1000 7- to 12-year-olds found that pet attachment of any kind encouraged compassion and positive attitudes toward animals, which promoted better well-being for both the child and the pet. Children with dogs scored the highest for pet attachment, and the study notes that "dogs may help children to regulate their emotions because they can trigger and respond to a child's attachment related behavior." And, of course, only one pet will happily play fetch with a toddler.

A version of this story originally ran in 2015.

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