Hammerhead Flatworms: Interesting and Unusual Land Planarians
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.
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 (The subdamily Bipaliinae contains the hammerhead flatworms.)
The term "land planarian" is often used to refer to hammerhead flatworms. This isn't wrong, but it should be noted that the Geoplanidae or land planarian family includes both hammerhead flatworms and similar animals that don't have a head with a semilunar shape.
Planarians and Hammerhead Flatworms
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. It has many features that resemble those of the genus Planaria.
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. They are often longer than slugs and earthworms and frequently prey on these animals. The information about land planarians in this article applies to hammerhead flatworms.
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.
An Aquatic Planarian Feeding
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 animal and is often found near the centre of the body. It also serves as an anus. The pharynx is a strong, muscular tube that is extended through the mouth in order to engulf food and then withdrawn into the body. The pharynx is continuous with the intestine inside the planarian's body. It passes the food that it has engulfed to the intestine, where digestion takes place. The nutrients then move into the animal's cells.
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.
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.
Human kidneys contain tiny filtering units called nephrons. These remove certaiin 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 in nephrons and body fluid in protonephridia) enters the tubule in the filtering structure first. 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.
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.
Introduced Hammerhead Flatworms
There are four genera in the hammerhead flatworm subfamily: Bipalium, Novibipalium, Diversibipalium, and Humbertium. The animals have been found in the United States as well as in parts of Europe. Researchers are afraid that the flatworms may become harmful in these areas.
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. The introduced animals (those outside their native region) have been found in greenhouses, plant nurseries, garden centres, and the wild in their new countries.
The Texas Invasive Species Institute website referenced below gives quite a long list of states where Bipalium kewense is found in the wild or in greenhouses. At the moment, the institute says that the community effects of the animal's presence are unknown.
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 above.
Giant Hammerhead Flatworms in France
In May 2018, news reports about five species of giant hammerhead flatworms living 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.
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.
Potential Effects of the Introduced Animals
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. Two benefits of their activity are their tunneling enables more air to enter the soil and the castings that they release fertilize the soil.
Introduced 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.
- 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, University of Hawaii
- Planarian “kidneys” go with the flow from the eLife journal
- Toxin production in terrestrial flatworms from Oxford University Press
- Giant worms chez moi! Hammerhead flatworms (Platyhelminthes, Geoplanidae, Bipalium spp., Diversibipalium spp.) in metropolitan France and overseas French territories from the PeerJ journal
- Hammerhead flatworms are invading France from the Smithsonian Magazine
- Bipalium kewense in the United States from the Texas Invasive Species Institute
Questions & Answers
I have a recording of one Hammerhead Flatworm and pictures of two more in central Arkansas. My state has not been listed anywhere on any site. Is that important information to share?
Yes, I think it is. I think researchers would be very interested in your discoveries. Perhaps you could contact a biologist at your nearest college or university or contact a scientist who studies land planarians and works at another institution. University websites and research institutions often list the email addresses of different departments, which could be helpful for you.Helpful 2
© 2018 Linda Crampton