Hammerhead Flatworms and Land Planarians: Facts and Photos

Updated on May 29, 2018
AliciaC profile image

Linda Crampton is a writer and teacher with an honors degree in biology. She loves to study nature and write about animals and plants.

A hammerhead flatworm (Bipalium sp.) in Malaysia
A hammerhead flatworm (Bipalium sp.) in Malaysia | Source

Flatworms That Live on Land

Hammerhead flatworms are predatory land animals with a narrow, elongated body and an unusual head shaped like a half moon or a pickaxe. They are native to Asia. Giant hammerhead flatworms are impressive animals that can reach a length of one to three feet. The animals have been introduced to the United States, where they are considered to be invasive. They belong to a group of organisms known as land planarians.

Some people may be familiar with planarians from their study of biology in school. School specimens are generally small animals that live in fresh water. They are popular because of their amazing ability to regenerate missing body parts. If they’re cut into pieces, each piece generally regenerates tissue and becomes a complete organism. Land planarians can also replace missing parts of their body, although perhaps to a more limited extent than their freshwater relatives. They are fascinating in many ways.

A hammerhead flatworm
A hammerhead flatworm | Source

Phylum Platyhelminthes

Land planarians and aquatic flatworms belong to the phylum Platyhelminthes. The name of the phylum comes from two Greek words—Platy, meaning flat, and helminth, meaning worm. As the name suggests, members of the phylum have flat or somewhat flattened bodies. Land planarians are currently classified as follows.

  • Phylum Platyhelminthes
  • Class Rhabditophora
  • Family Geoplanidae (All land planarians)
  • Four subfamilies within the Geoplanidae family (one of which contains the hammerhead flatworms)

The Family Geoplanidae: Land Planarians

The term "planarian" has an imprecise meaning. One freshwater species of flatworm belongs to the genus Planaria. This organism and freshwater flatworms of other genera that resemble it are referred to as planarians. The animals are small, have flat bodies, and have an intriguing ability to regenerate. The planarian that is often used by school and university labs is Dugesia (or Giradia) tigrina, which is brown in colour.

Land planarians are recognized as relatives of the aquatic planarians due to their internal structure and their ability to regenerate. Despite their membership in the flatworm phylum, their body isn't completely flat. It's flatter than that of an earthworm, however, especially in the second half of the body.

Hammerhead flatworms are interesting members of the land planarian family. They produce mucus and have a glistening appearance. The animals are sometimes confused with slugs. They are also known as hammerhead slugs, hammerhead worms, and arrowhead worms. Unlike slugs, they don't have tentacles at the front of their head, and unlike earthworms, their bodies aren't segmented. Hammerhead flatworms are often longer than slugs and earthworms and frequently prey on these animals.

The video below shows a freshwater planarian and its feeding process. Land planarians also extend their pharynx out of their body to feed, like their aquatic relatives.

Respiration and Digestion

A land planarian is simpler internally than a slug or an earthworm. It lacks a skeleton, a respiratory system, a circulatory system, and an anus. Despite these limitations, the animal is a very capable hunter.

Oxygen is absorbed through the surface of the animal and carbon dioxide released through it. The gases moves by diffusion both through the body surface and within the body.

The mouth is located on the underside of the flatworm and is often located near the centre of the body. It also serves as an anus. The pharynx is a strong, muscular tube that connects the mouth to the intestine and is capable of being turned inside out. It extends through the mouth in order to engulf food. The pharynx then passes the food to the intestine, which is branched. Here digestion is completed. The nutrients from the food diffuse out of the intestine and into the body cells.

Cell body of a human neuron with an axon covered with myelin extending away from it
Cell body of a human neuron with an axon covered with myelin extending away from it | Source

The dendrites of a neuron (or nerve cell) transmit the nerve impulse to the cell body. The cell body contains the nucleus of the neuron. A fibre known as an axon extends from the cell body and transmits the nerve impulse towards the next neuron.

Cerebral Ganglion and Sense Organs

The members of the phylum Platyhelminthes are bilaterally symmetrical. They are the simplest animals that show signs of a central nervous system. Flatworms have the beginnings of a brain in their head. This specialization is known as cephalization.

A land planarian doesn't have a true brain. It does have a cerebral ganglion in the head region that serves some of the functions of a simple brain, however. A ganglion is a collection of cell bodies from different neurons. The cerebral ganglion of a planarian is connected to nerves in the animal's body. A nerve is a bundle of axons.

A land planarian has sense organs on the surface of its body, including eyespots on its head. These can distinguish light areas from dark ones but can't form an image. Sensory receptors on the animal's surface detect a variety of chemicals in the environment. Other receptors detect touch.

The head of a hammerhead flatworm often looks as though it's rippling as the animal travels. The head is sensing the environment with its chemoreceptors as it ripples. The flatworm can track its prey via secretions that the prey releases.

Bipalium kewense
Bipalium kewense | Source

A land planarian has a creeping sole on its undersurface. The area is covered with fine, moveable hairs known as cilia that help the animal to move. The animal appears to glide over the ground.

Excretion

Human kidneys contain tiny filtering units called nephrons. These remove certain substances from the blood and send them to the urinary bladder to be excreted as urine. Land planarians have simpler protonephridia, which have a somewhat similar function to nephrons. They remove substances from body fluid and release them to the outside world through pores on the body surface.

Nephrons and protonephridia have another similarity in function. The fluid (blood plasma or body fluid) enters the tubule in the filtering structure first and then valuable substances are reabsorbed so that they aren't lost from the body.

The protonephridia have cells containing flagella (slender, hair-like structures). Flagella are quite similar to cilia but are longer. The flagella in the protonephridia move back and forth rapidly. This movement creates a current of fluid leading to the exit pore on the surface of the body. The cells containing the flagella are referred to as flame cells because the moving flagella reminded early observers of a flickering flame.

Hammerhead Flatworm Eating an Earthworm

Life of a Land Planarian

Though land planarians are terrestrial, they lose water quite easily and need to keep their body moist. They are generally seen at night or during the day when the atmosphere is humid.

Land planarians are carnivores. Most are predators, but some are said to be scavengers. At least some species actively hunt their prey. They eat earthworms, snails, slugs, insects, woodlice, millipedes, and other invertebrates. Snails, earthworms, and perhaps the other prey are partially digested and then taken into the body.

Multiple factors enable a land planarian to catch its prey. One is physical force created by both its pharynx and its body. The body can quickly wrap around the prey, trapping it. Other helpful features are the adhesive mucus and digestive enzymes released by the pharynx.

In 2014, a research team found that the hammerhead flatworms Bipalium kewense and Bipalium adventitium produce tetrodotoxin. This substance has also been found in more advanced animals and is a powerful neurotoxin. It may help the flatworms to subdue their prey, though further research is needed to prove this.

Two land planarians (Bipalium sp.) mating in Malaysia
Two land planarians (Bipalium sp.) mating in Malaysia | Source

Reproduction

Reproduction occurs by fragmentation and by sexual reproduction. In fragmentation, the animal grips hold of an object with the tip of its tail. The head then moves away, splitting the animal into two fragments. These both survive and regenerate the missing parts. In at least two species, the tail grows a new head in seven to ten days.

Researchers know that regeneration in planarians depends on the action of stem cells, which are common in the animal's body, but more research is needed to fully understand the process. Stem cells are unspecialized cells that have to ability to produce specialized ones under certain circumstances.

Land planarians are hermaphrodites, which means they have both male and female reproductive organs in their body. When the animals mate, sperm passes from each animal to the other one. Fertilization is internal. The worms lay the fertilized eggs inside cocoons. The eggs take around three weeks to hatch.

Bipalium adventitium
Bipalium adventitium | Source

Hammerhead Flatworms

Hammerhead flatworms belong to the subfamily Bipaliinae within the family Geoplanidae. There are four genera in the hammerhead flatworm subfamily: Bipalium, Novibipalium, Diversibipalium, and Humbertium. The animals have established themselves in the United States as well as in parts of Europe.

It's thought that terrestrial flatworms are being transported to countries outside their natural habitats on horticultural plants and in soil around the roots of potted plants. They've been found in greenhouses, plant nurseries, garden centres, and the wild in their new countries.

There's no scientific definition of a "giant" hammerhead flatworm. The term is used for the longest animals in the subfamily. Though they are long animals, they are also slender, as can be seen in the animal shown in the video below.

Giant Hammerhead Flatworms in France

In May 2018, news reports about five species of invasive giant hammerhead flatworms in France were widely published. One reason why the reports were popular was because they claimed that the flatworms had been in France for twenty years without being discovered by scientists, which seemed strange.

Scientists became aware of the flatworms as a result of an amateur naturalist's actions. In 2013, Pierre Gros took a photo of a hammerhead flatworm in France and sent it to experts in his area, who in turn sent it to a scientist named Jean-Lou Justine. Justine thought that the photo was a joke. The naturalist sent him two more photos, each showing a different type of hammerhead flatworm. Although Justine still thought the idea that the animals were living in France was a joke, he decided to investigate the situation. He eventually discovered that the photos depicted a real situation.

Justine began an advertising campaign asking for people to send him photos of land planarians in France. Many people responded, including one family who had a VHS tape of a hammerhead flatworm recorded in 1999. The family had taken care of the tape due to the strange nature of the animal that they filmed. The flatworms do indeed seem to have been in France for around twenty years, though it's possible that they appeared about twenty years ago, disappeared, and then reappeared later.

Diversibipalium multilineatum
Diversibipalium multilineatum | Source

Bipalium kewense, Diversibipalium multilineatum, and an unnamed, black species of Diversibipalium have been found in France. Bipalium vagum and a blue species of Diversibipalium have been found in French territories.

Effects of the Invaders

Hammerhead flatworms are unusual animals for those of us who live in North America, but they are interesting creatures. In Europe there are fears that they could harm the earthworm population if they become too numerous. This is worrying because earthworms are beneficial for soil. For example, their tunneling enables more air to enter the soil and the castings that they release fertilize the soil.

Invasive species often create problems when they leave their natural habitat and enter a region with a different environment and the absence of their usual predators. At the moment, however, the fears about hammerhead flatworms haven't materialized, at least in North America and France. They are curious animals that are interesting to observe.

References

Information about two land planarians from the University of Florida

Facts about the shovel-headed garden worm (Bipalium kewense) from the Australian Museum

Information about the phylum Platyhelminthes and other worm-like animals from Exploring Our Fluid Earth

Issigonis, M., & Newmark, P. A. (2015). Planarian “kidneys” go with the flow. eLife, 4, e09353. http://doi.org/10.7554/eLife.09353

Toxin production in terrestrial flatworms from Oxford University Press

Justine J, Winsor L, Gey D, Gros P, Thévenot J. (2018) Giant worms chez moi! Hammerhead flatworms (Platyhelminthes, Geoplanidae, Bipalium spp., Diversibipalium spp.) in metropolitan France and overseas French territories. PeerJ 6:e4672 https://doi.org/10.7717/peerj.4672

Hammerhead flatworms are invading France from the Smithsonian Magazine

Questions & Answers

    © 2018 Linda Crampton

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      • AliciaC profile imageAUTHOR

        Linda Crampton 

        4 weeks ago from British Columbia, Canada

        That's an interesting idea, Dianna! Thanks for the comment.

      • teaches12345 profile image

        Dianna Mendez 

        4 weeks ago

        Upon reading, the thought came to me that they were miniature boa constrictors. Yet another interesting article on animal life new to me!

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        5 weeks ago from British Columbia, Canada

        Hi, rdsparrowriter. I can understand why pranks from relatives have made you wary of certain animals. Pranks can be both funny and annoying! Thank you for the visit and the comment.

      • rdsparrowriter profile image

        rdsparrowriter 

        5 weeks ago

        They look so slippery and shiny. I'm not really good at recognizing animals. For me I'm not a big fan of certain animals thanks to my cousin brothers pranks :( However thanks to your dedication and information I was fascinated to get to know about their existence :) Thank you for educating me :)

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        5 weeks ago from British Columbia, Canada

        Hi, Hari. Thanks for the comment. The animals reportedly live in some parts of India, but I don't know how numerous they are.

      • shprd74 profile image

        Hari Prasad S 

        5 weeks ago from Bangalore

        A very interesting article. I liked it very much. Informative. I haven't seen these flatworms here in India though.

        - hari

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        5 weeks ago from British Columbia, Canada

        Thank you for the comment, Adrienne. The flatworms are certainly odd creatures. I think they're worth studying.

      • alexadry profile image

        Adrienne Janet Farricelli 

        5 weeks ago from USA

        I love how you go in depth on so many interesting topics. I had to look twice at the picture, never so such an oddly looking flatworm, but then I never went to Asia, where it sounds like most of them are found.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        6 weeks ago from British Columbia, Canada

        I appreciate your visit and your comment very much, Liz.

      • Eurofile profile image

        Liz Westwood 

        6 weeks ago from UK

        This is a very detailed article, packed with information and interesting illustrations. Earthworms predominate in our area.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        6 weeks ago from British Columbia, Canada

        Thanks for sharing your experience, Peggy. It's interesting that you see hammerhead flatworms in Houston. I hope they don't proliferate, too!

      • Peggy W profile image

        Peggy Woods 

        6 weeks ago from Houston, Texas

        I have spotted these in our Houston garden and thought that they were just an unusual type of earthworm. Thanks for the education regarding these hammerhead flatworms. The ones here are brown in color not too different from the earthworms except for that unusually shaped head. I hope that they do not proliferate and become harmful to our earthworm population!

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        6 weeks ago from British Columbia, Canada

        It sounds wonderful!

      • manatita44 profile image

        manatita44 

        6 weeks ago from london

        Isle of Spice and everything nice.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        6 weeks ago from British Columbia, Canada

        I think it would be an interesting place to visit.

      • manatita44 profile image

        manatita44 

        6 weeks ago from london

        We have a lot in the villages. Cooler there with so many trees. Alas! I don't have your savvy, so I don't know what is what.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        6 weeks ago from British Columbia, Canada

        Hi, Manatita. Thanks for commenting. Hammerhead flatworms can and do live in warm climates, but since they need to keep moist I doubt whether they would do well in a very hot one. They've been found in some parts of the Caribbean, but I don't know whether their habitat includes Grenada.

      • manatita44 profile image

        manatita44 

        6 weeks ago from london

        Fascinating and also scary stuff! They worry me. Lol.

        A pretty detailed account. I wonder if we have them home in Grenada? Can they live in hot temperatures?

        A great video!

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        6 weeks ago from British Columbia, Canada

        Hi, Jackie. I find the flatworms fascinating, but I realize that some people may feel uneasy about their slime. I appreciate your visit, especially since you don't like slimy animals!

      • Jackie Lynnley profile image

        Jackie Lynnley 

        6 weeks ago from The Beautiful South

        Fascinating and repulsive at the same time, Linda. Guess I am just a girl through and through when it comes to anything slimy, no matter how pretty it is!

        Thanks for another great lesson.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        6 weeks ago from British Columbia, Canada

        Hi, Bede. Yes, the animals do hitch a ride on plants. I hope they remain an interesting but relatively harmless addition to the wildlife of North America.

      • Bede le Venerable profile image

        Bede 

        6 weeks ago from Minnesota

        Linda, I enjoyed reading about these worms. They are quite interesting little creatures, especially in their reproductive life. Introducing creatures into a foreign environment is often a bad scenario, but it sounds like these worms hitched on plants.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        6 weeks ago from British Columbia, Canada

        Hi, Heidi. Yes, I think they're very interesting creatures. Nature has a lot of surprises for us!

      • heidithorne profile image

        Heidi Thorne 

        6 weeks ago from Chicago Area

        Dang! You just keep introducing us to the world we live in, but don't know. Aren't these interesting creatures! Thanks, as always!

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        6 weeks ago from British Columbia, Canada

        I hope someone finds a use for their abilities, too, Peg. It would be great if they helped humans in some way.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        6 weeks ago from British Columbia, Canada

        Sorry about the potential nightmares, Larry! Thanks for the visit.

      • PegCole17 profile image

        Peg Cole 

        6 weeks ago from Dallas, Texas

        Glad to read your explanation of this strange creature and their interesting phenomenon of regeneration. As creepy as they are, I really hope someone finds a positive use for their unusual abilities.

      • Larry Rankin profile image

        Larry Rankin 

        6 weeks ago from Oklahoma

        Just so freaky looking!

        Thanks for fueling my nightmares, lol.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        6 weeks ago from British Columbia, Canada

        Hi, Flourish. Yes, I think studying their ability to regenerate could be very helpful for us. It's possible that some of the knowledge gained from studying planarian stem cells could apply to human ones.

      • FlourishAnyway profile image

        FlourishAnyway 

        6 weeks ago from USA

        I’m still amazed at their size. Their ability to regenerate should teach us something.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        6 weeks ago from British Columbia, Canada

        I love your description of the animals, Bill! I appreciate your visit and your comment.

      • billybuc profile image

        Bill Holland 

        6 weeks ago from Olympia, WA

        Absolutely amazing! I have never heard of them but man, they are really cool in a destructive, predatory sort of way. Thanks, Linda!

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        6 weeks ago from British Columbia, Canada

        Hi, Chris. Thank you very much for the comment. An article that I read recently actually used the term "voracious predator" to refer to one flatworm that ate earthworms. We'll have to see what happens in the future. I hope the problem doesn't become serious.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        6 weeks ago from British Columbia, Canada

        I agree, Mary. I think invasive species could become an increasingly serious problem over time. The potential effects are worrying.

      • cam8510 profile image

        Chris Mills 

        6 weeks ago from Missoula, Montana through August 2018

        It would seem that there would need to be a lot of hammerhead flatworms, or that they are voracious eaters, to threaten the earthworm population. But if the experts are concerned, then it must be a legitimate issue. You've mentioned the necessity of earthworms in the production of new soil. This is fascinating and informative. You never disappoint with your articles, Linda.

      • aesta1 profile image

        Mary Norton 

        6 weeks ago from Ontario, Canada

        With the world now heavily interacting, many invasive species will be spread in areas where they have no natural predators. I wonder how widely this spread will happen and what its effects will be.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        6 weeks ago from British Columbia, Canada

        Thank you very much, Nell. I like the colours, too. Some of the flatworms are very attractive.

      • Nell Rose profile image

        Nell Rose 

        6 weeks ago from England

        How fascinating! I have never seen these before. And the colors are amazing! great information as always.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        6 weeks ago from British Columbia, Canada

        I appreciate your comment, Larry. The flatworms are intriguing creatures.

      • Larry Slawson profile image

        Larry Slawson 

        6 weeks ago from North Carolina

        Very interesting. I'm with Barbara Kay, I have never seen one of these creatures before. Thanks for sharing.

      • AliciaC profile imageAUTHOR

        Linda Crampton 

        6 weeks ago from British Columbia, Canada

        Hi, Barbara. Thanks for the visit. I can understand why the animals look scary to someone who's not familiar with them. They're not harmful to humans, but they are unusual.

      • Barbara Kay profile image

        Barbara Badder 

        6 weeks ago from USA

        I have never seen one of these creatures, not even in a photo. Our biology class in high school mainly covered the human body and just a small amount of animal life. I hope I never see one, since they look kind of scary.

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