The flatworms (Platyhelminthes, Greek "platy"': flat; "helminth": worm) are a phylum of relatively simple soft-bodied invertebrate animals. With about 20,000 species they are largest phylum of Acoelomates. Flatworms are found in marine, freshwater, and even damp terrestrial environments. Most are free-living forms, but many are parasitic on other animals. There are four classes: Trematoda, Cestoda, Monogenea, and Turbellaria.
The body is ribbon-shaped, soft-bodied and is flattened dorso-ventrally, from top to bottom. They are bilaterally symmetrical and are triploblastic, meaning they have three germ layers; the outer ectoderm, an inner endoderm and a mesoderm between the two. Turbellarians generally have a ciliated epidermis, while cestodes and trematodes are covered with a cuticle.
There is also no true body cavity except the gut and hence they are Acoelomates. The interior of the acolelomate body is filled with somewhat loosely spaced mesodermal tissue called parenchyma tissue. This is not a true circulatory or respiratory system, but extracellular body fluids (interstitial fluids) do percolate between cells to help distribute nutrients, gases and waste products.
Usually the digestive tract has one opening, but in particularly long worms or those with highly branched guts, there may be one or more anuses. In acoelomate flatworms, now thought to be unrelated to the Platyhelminthes, the gut is absent or non-permanent. Since most flatworms have only one opening to their digestive tract, they can't feed, digest, and eliminate undigested particles of food simultaneously, as most animals with tubular guts can. This blind-ended gastrovascular cavity functions similarly to that of the Cnidaria.
They are, however, more complex than cnidarians in that they possess numerous organs, and are therefore said to show an organ level or 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 upper end of the gut causes a strong sucking force allowing flatworms to ingest their food and tear it into small bits. The gut is branched and extends throughout the body, functioning in both digestion and transport of food. Flatworms exhibit an undulating form of locomotion.
Flatworm reproduction is hermaphroditic, meaning each individual produces eggs and sperm. Depeding on species and age, individuals can range in size from almost microscopic to over 20 m long (some tapeworms can attain this length).
Flatworms were formerly considered 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. The true flatworms form a monophyletic group that developed from more complex ancestors, and is generally considered to belong among the Lophotrochozoa. The traditional classifications of flatworms is primarily based on differing degrees of parasitism and divided into three monophyletic classes,:
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 extreme damage to humans and other animals.
- 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 Feb 2005] (http://www.infoplease.com/ce6/sci/A0839338.html)