The earth is filled with many types of worms, and the term “planarian” can represent a variety of worms within this diverse bunch of organisms. The slideshow below highlights fun facts about planarians from Oné Pagán’s book, The First Brain: The Neuroscience of Planarians, and provides a glimpse of why scientists like Pagán study these fascinating creatures.
Planarians in Popular Culture
Planarians are not only test subjects for scientists to study. These are also creatures of popular culture, making appearances in several movies and TV shows. For example, in an episode of Fringe, Dr. Bishop offers Agent Dunham a smoothie with chopped pieces of planarians—a gesture meant to pay homage to the memory transfer experiments conducted by McConnell. Planarians have also appeared in the first movie of the Twilight saga, in addition to episodes of The Big Bang Theory and Dr. Who.
Triclads are small worms. They do not typically exceed one inch in length, but many of these worms are even smaller than that. Though these planarians can usually be seen with the naked eye, they are often studied under a microscope for closer observation. The two distinctive bumps that you can see on both sides of a planarian’s head, even without a microscope, are called auricles. Though they are commonly mistaken for ears, auricles do not pick up sounds in the environment, and instead contain many chemoreceptors that help planarians sense both nourishing and toxic substances in their surrounding environment.
In taxonomic terms, planarians belong to a large class of organisms called Vermes, the Latin term for “worm.” Platyhelminthes, or “flatworms,” represent a phylum within the class of Vermes, though the Platyhelminthes are broken down into four additional categories. One of these categories includes the Turbellarians—flatworms that are free-living and non-parasitic. Turbellarians are then arbitrarily distinguished based on their size, creating two further divisions: the microturbellarians (worms that are shorter than 1 mm) and macroturbellarians (worms that are longer than 1 mm). There are two types of macroturbellarians, the triclads and polyclads. Though “planarian” is a general term used to describe many types of flatworms, it is most often used in reference to triclads.
Planarians possess the incredible capability to regenerate cells that are damaged or removed. Though the scope of regeneration varies from species to species, many planarians are capable of full regeneration. This means that if you chop up a planarian into several pieces—the current record is 279 sections—each piece can regenerate into a full grown worm, assuming that this piece of the worm is placed in an environment with adequate nourishment. Even the isolated tip of a tail from a planarian can develop into a full grown worm that possesses a brain and central nervous system.
Planarians as Model Organisms
In the early 20th century, scientists began looking for organisms with which they could test the principles of Mendelian genetics. Most people are aware that fruit flies (Drosophila melanogaster) became a common test subject because of their short lifespans and ability to produce large quantities of offspring in a short period of time. However, planarians also became a great model organism for scientists to use. Though the planarian nervous system is simple, these flatworms display an immense array of complex behaviors—making planarians an ideal candidate with which scientists can study how the brains of more complex organisms, such as humans, function.
Planarians are an ancient species. But like other invertebrates, it’s hard for planarians to fossilize because their bodies lack hard bones. Planarians’ tendency to autolyze—or dissolve head first—when they die makes the process of fossilization even more difficult. Though the fossil record for these organisms is a bit scarce, scientists have identified a fully intact turbellarian fossil from the Eocene period about 40 million years ago. Scientists have also found what they interpret as fossilized flatworm tracks from the Permian period (300 million years ago).
Images: The first five photos in this slideshow have been used courtesy of Dr. Masaharu Kawakatsu. Photo six is copyrighted (2003) by the National Academy of Sciences, USA and has been used with permission.