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Dictionary Definition

flatworm n : parasitic or free-living worms having a flattened body [syn: platyhelminth]

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  1. Any of very many parasitic or free-living worms, of the phylum Platyhelminthes, having a flattened body with no skeleton or body cavity.

Extensive Definition

The flatworms (Phylum Platyhelminthes from the Greek platy, meaning "flat" and helminth, meaning worm) are a phylum of relatively simple soft-bodied invertebrate animals. With about 25,000 known species they are the largest phylum of acoelomates. Flatworms are found in marine, freshwater, and even damp terrestrial environments. A troublesome terrestrial example is the New Zealand flatworm, Arthurdendyus triangulatus, which rapidly colonized large areas of Ireland and Scotland since its unintentional introduction in the 1960s and has since destroyed most of the indigenous earthworms . Most flatworms are free-living, but many are parasitic. There are four classes: Trematoda (flukes), Cestoda (tapeworms), Monogenea, and Turbellaria.


The flatworm’s cephalized soft body is ribbon-shaped, flattened dorso-ventrally (from top to bottom), and bilaterally symmetric. Flatworms are the simplest triploblastic animals with organs. This means their organ systems form out of three germ layers: an outer ectoderm and an inner endoderm with a mesoderm between them. Turbellarians generally have a ciliated epidermis. There is also no true body cavity (coelom) except the gut; hence, flatworms are classified as acoelomates. The interior of the acoelomate body is filled with somewhat loosely spaced mesodermal tissue called parenchyma tissue.
Flatworms have a hydrostatic skeleton, which consists of a water filled body cavity that is controlled by muscles.
Flatworms exhibit an undulating form of locomotion.
Depending on species and age, individuals can range in size from almost microscopic to over 20 m long. The longest ever recorded flatworm was a tapeworm over 90 ft (27 m) long.


The haptor is a part of Platyhelminthes used to attach to its host. These differ between the prohaptor which is on the anterior end of the body and the opisthaptor, which is part of the posterior end of the body.
The prohaptor has various adhesive and feeding structures. In some species, the prohaptor may have a number of cephalic or head glands that secrete a sticky adhesive substance, and shallow muscular suckers, all used for attachment. In other species there is an oral sucker, with various degrees of muscularisation that surrounds the mouth.
The opisthaptor is primarily responsible for the attachment of the monogeneans to the host. The morphology of the opisthaptor is highly variable. It may have suckers in various degrees of development, large hooks called anchors (or hamuli), small hooks that are remnants from the larval stage, or complex clamps that may be either muscular or sclerotised.

Circulation and nervous system

There is no true circulatory or respiratory system, but like all other animals, flatworms do take in oxygen. Extracellular body fluids (interstitial fluids) percolate between cells to help distribute nutrients, gases, and waste products.
Flatworms respire at their integument; gasses diffuse directly across their moist outer surface. This type of system is called integumentary exchange.
However, flatworms do have a bilateral nervous system; they are the simplest animals to have one. Two cordlike nerves branch repeatedly in an array resembling a ladder. The head end of some species even has a collection of ganglia acting as a rudimentary brain to integrate signals from sensory organs such as eyespots.


Usually the digestive tract has one opening, so the animal can feed, digest, and eliminate undigested particles of food simultaneously, as most animals with tubular guts are able to do. This blind-ended gastrovascular cavity functions similarly to that of the Cnidaria. However, in a few particularly long flatworms or those with highly branched guts, there may be one or more anuses. A small group where the gut is absent or non-permanent, called acoel flatworms, appear to be unrelated to the other Platyhelminthes (see below).They eat rarely, maybe a few times per year(once or twice). Their diet includes small bugs found in soils and other types of worms, smaller then itself.
Despite the simplicity of the digestive chamber, they are significantly more complex than cnidarians in that they possess numerous organs, and are therefore said to show an organ level of organization. Mesoderm allows for the development of these organs, and true muscle. Major sense organs are concentrated in the front end of the animals for species who possess these organs.
Muscular contraction in the gut causes a strong sucking force which allows flatworms to ingest food.


Flatworm reproduction is hermaphroditic, meaning each individual produces eggs and sperm. When two flatworms mate, they exchange sperm so both become fertilized. Some flatworms, such as Pseudobiceros hancockanus engage in penis fencing, in which two individuals fight, trying to pierce the skin of the other with their penises; the first to succeed inseminates the other, which must then carry and nourish the fertilized eggs. Flatworms usually do not fertilize their own eggs.
Turbellarians classified as planarians (usually freshwater, non-parasitic) can also reproduce asexually by transverse fission. The body constricts at the midsection, and the posterior end grips a substrate. After a few hours of tugging, the body rips apart at the constriction. Each half grows replacements of the missing pieces to form two whole flatworms. This also means that if one of these planarian flatworms is cut in half, each half will regenerate, forming two separate, fully-functioning flatworms.


Flatworms were formerly consider to be basal among the protostomes. Molecular evidence suggests that this is only true of the orders Acoela and Nemertodermatida, which are thus given their own phylum Acoelomorpha. These findings, however, are still not accepted by all biologists. The systematic position of Catenulida seems uncertain, although Donoghue and Cracraft would place it as a sister group to all other non-Acoelomorpha flatworms. Xenoturbella was at first believed to be a flatworm as well, but it is now obvious that it belongs in its own phyla. The remaining and true flatworms form a monophyletic group that developed from more complex ancestors, and grouped with several other phyla as the Platyzoa. The traditional classifications of flatworms is primarily based on differing degrees of parasitism and divided into three monophyletic classes:
  • Trematoda - flukes, probably paraphyletic to Cestoda.
  • Cestoda - tapeworms
  • Monogenea - ectoparasitic flukes with simpler life cycles than Trematode flukes. They live an exclusively parasitic existence.
The remaining flatworms are grouped together for convenience as the class Turbellaria, now comprising the following orders:
Most of these groups include free-living forms. The flukes and tapeworms, though, are parasitic, and a few cause massive damage to humans and other animals.

Biochemical memory experiments

In 1955, Thompson and James V. McConnell conditioned planarian flatworms by pairing a bright light with an electric shock. After repeating this several times they took away the electric shock, and only exposed them to the bright light. The flatworms would react to the bright light as if they had been shocked. Thompson and McConnell found that if they cut the worm in two, and allowed both worms to regenerate each half would develop the light-shock reaction. In 1962, McConnell repeated the experiment, but instead of cutting the trained flatworms in two he ground them into small pieces and fed them to other flatworms. Incredibly these flatworms learned to associate the bright light with a shock much faster than flatworms who had not been fed trained worms.
This experiment intended to show that memory could perhaps be transferred chemically. The experiment was repeated with mice, fish, and rats, but it always failed to produce the same results, . The perceived explanation was that rather than memory being transferred to the other animals, it was the hormones in the ingested ground animals that changed its behaviour. McConnell believed that this was evidence of a chemical basis for memory, which he identified as memory RNA. McConnell's results are now attributed to observer bias. No double-blind experiment has ever reproduced his results.


  • Campbell, Neil A., Biology: Fourth Edition (Benjamin/Cummings Publishing, New York; 1996; page 599) ISBN 0-8053-1957-3
  • Crawley, John L., and Kent M. Van De Graff. (editors); A Photographic Atlas for the Zoology Laboratory: Fourth Edition) (Morton Publishing Company; Colorado; 2002) ISBN 0-89582-613-5
  • Naganuma, Kenneth H. (PhD) ; Lab handout "Acoelomate Flatworms, Phylum Platyhelminthes", handed out in Fall 1997 (adapted GNU Free Documentation Licensed text: Permission granted in February 2005).
  • The Columbia Electronic Encyclopedia, 6th ed. (Columbia University Press; 2004) [Retrieved 8 February 2005]
  • Evers, Christine A., Lisa Starr. Biology:Concepts and Applications. 6th ed. United States:Thomson, 2006. ISBN 0-534-46224-3.
flatworm in Bosnian: Pljosnati crvi
flatworm in Bulgarian: Плоски червеи
flatworm in Catalan: Platihelmint
flatworm in Czech: Ploštěnci
flatworm in Welsh: Llyngyren ledog
flatworm in Danish: Fladorme
flatworm in German: Plattwürmer
flatworm in Estonian: Lameussid
flatworm in Modern Greek (1453-): Πλατυέλμινθες
flatworm in Spanish: Platyhelminthes
flatworm in Esperanto: Platvermo
flatworm in French: Platyhelminthes
flatworm in Croatian: Plošnjaci
flatworm in Italian: Platyhelminthes
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flatworm in Georgian: ბრტყელი ჭიები
flatworm in Latin: Platyhelminthes
flatworm in Latvian: Plakantārpi
flatworm in Lithuanian: Plokščiosios kirmėlės
flatworm in Macedonian: Сплескани црви
flatworm in Dutch: Platwormen
flatworm in Japanese: 扁形動物
flatworm in Norwegian: Flatormer
flatworm in Norwegian Nynorsk: Flatmakk
flatworm in Occitan (post 1500): Platyhelminthes
flatworm in Polish: Płazińce
flatworm in Portuguese: Platelmintos
flatworm in Romanian: Vierme lat
flatworm in Quechua: Pampa kuru
flatworm in Russian: Плоские черви
flatworm in Simple English: Flatworm
flatworm in Slovak: Ploskavce
flatworm in Slovenian: Ploski črvi
flatworm in Serbian: Пљоснати црви
flatworm in Serbo-Croatian: Pljosnati crvi
flatworm in Finnish: Laakamadot
flatworm in Swedish: Plattmaskar
flatworm in Telugu: ప్లాటీహెల్మింథిస్
flatworm in Thai: หนอนตัวแบน
flatworm in Turkish: Yassı solucanlar
flatworm in Ukrainian: Плоскі черви
flatworm in Chinese: 扁形动物门
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