Flatworms are small, multicelled animals with elongated bodies that have clearly defined anterior (front) and posterior (rear) ends. These worms are bilaterally symmetrical, meaning that their two sides reflect each other. They usually have a recognizable head, which houses gravity and light-receptive organs, and eye spots. They lack circulatory and respiratory systems and have only one opening that serves both as their anus and mouth. Most flatworm species live in fresh and marine waters, although some live on land.
Their soft, flattened bodies are composed of three layers—the ectoderm, endoderm, and mesoderm. They may be covered by a protective cuticle or by microscopic hairs, called cilia. Their internal organs are comprised of a nervous system, usually hermaphrodite sexual organs, and an excretory system.
As members of the phylum Platyhelminthes, flatworms belong to four classes: Turbellaria, Monogenea, Trematoda, and Cestoidea. Within these four classes, there are hundreds of families and some 10,000 species, including animals with common names like free-living flatworms, parasitic flatworms, tapeworms, and flukes.
Containing the most primitive flatworms, the class Turbellaria consists of nine orders and a total of about 3,000 species, most of which are free-living. While some species live in moist, dark areas on land, most live at the bottom of marine water. These flat-worms are found in all seas. While the aquatic species seldom grow more than 0.4-0.8 inches (1–2 cm) long, some land varieties can reach lengths of 19.7 inches (50 cm). The aquatic species have relatively flat, leaf–shaped bodies and are usually gray, brown, or black, although some species have a green tint. The turbel–laria’s head possesses one or more pair of eyes and tentacles. These flatworms are covered by microscopic hairs (cilia) that they beat continuously, creating turbulence in the water–an activity that gave them their name. Their cilia are important in their locomotion; they also crawl along the ground gripping it with sticky secretions from their glands.
Turbellarians are hermaphrodites, possessing the complex reproductive apparatus of both male and female. The fertilized eggs usually produce a small worm, although sometimes larvae are produced. The majority of turbellarians are carnivores.
Probably the most familiar Turbellarians are the planarians, soft–bodied, aquatic, flattened worms that appear to have crossed eyes and ear lobes. In fact, the crossed eyes are eye spots with which the worm can detect light. The lobes to each side are sensory and also are equipped with glands to secrete an adhesive substance used in capturing prey.
The single opening on the ventral (bottom) surface of the worm serves as both mouth and anus. Internally the worm has a complex, branching gut that courses nearly the full length of the body. Since the worm has no circulatory system, the elongated gut brings food to nearly all areas of the worm’s body. Planaria have no skeletal or respiratory systems.
These animals possess great powers of regeneration. If a planaria is cut in half, the front half will grow a new tail section and the rear half will generate a new head. If cut into thirds, the middle third will regrow a head and tail and the other two sections will regenerate as described.
Species in the class Monogenea are parasites, completing their life cycles within the body of a single living host, such as fish, frogs, turtles, and squid. Most of the 400 species in this class are ectoparasites, meaning that they cling to the outside of their host, for example, to the gills, fins, or skin of fish. Their bodies are covered by a protective cuticle and have adhesive suckers at each end. They eat by sucking blood through their mouths, which open beside their suckers.
Commonly known as flukes, there are over 6,000 species of flatworm in this class. Descended from the parasitic flatworm, flukes grow slightly larger, to about 0.8–1.2 inches (2–3 cm) long. A fluke must live in two or more hosts during its lifetime because its developmental needs are different than its adult needs. The first host is usually a mollusk and the final host— which houses the fluke during its mature, sexual stagmdash;is invariably a vertebrate, sometimes a human. In general, flukes lay tens of thousands of eggs to increase their offspring’s chances of survival.
Three families in this class contain blood flukes, those that live in the bloodstream of their hosts. Blood flukes, called schistosomes, are of particular importance to humans, since an estimated 200 million people are affected by them. Second only to malaria among human parasites, they usually do not kill their victims immediately; rather, they make their hosts uncomfortable for years until a secondary illness kills them.
As larvae, some species inhabit snails but, upon destroying their hosts’ livers, leave and swim freely for several days. They are then absorbed through the skin of a second host, such as a human, and live in veins near the stomach. There they mature and can live for 20 years or more. Unlike other species in the phylum, blood flukes have clearly defined gender.
Tapeworms are the dominant member of the class Cestoidea. They are ribbonlike, segmented creatures living in the intestines of their vertebrate hosts. There are a dozen orders in this class, most living in fish but two that use humans as hosts. Tapeworms cling to the intestinal wall of their hosts with suckers, hooks, or other adhesive devices. Having no mouth or gut, they receive their nourishment through their skin. Further, they have no type of sensory organs. White or yellowish in color, species in this class vary from 0.04 inches (1 mm) long to over 99 feet (30 m).
The broad fish tapeworm (Diphyllobothrium latum ), a large tapeworm present in humans, can illustrate the typical life of a tapeworm. As an adult, it attaches itself to the intestinal wall of the human host. Its body, composed of roughly 3,500 sections, probably measures 33–66 feet (10–20 m) long. At this point, it lays about one million eggs each day. Each egg, encased in a thick capsule so that it will not be digested by the host, leaves the host through its feces. When the egg capsule reaches water, an embryo develops and hatches. The larva swims until it is eaten by its first host, a minute crustacean called a copepod. The larva feeds on the copepod and grows. When a fish eats the copepod, the young tapeworm attaches itself to the fish’s gut. The tapeworm continues to grow and develop until the fish is eaten by a suitable mammal, such as a human. Only at this point can the tapeworm mature and reproduce.
The Illustrated Encyclopedia of Wildlife. London: Grey Castle Press, 1991.
Bilateral symmetry —The flatworm is divisible into two identical halves along a single line down the center of its body, called the sagittal plane.
Ectoderm —The skin covering the flatworm on the outside of its body.
Ectoparasites —Parasites that cling to the outside of their host, rather than their host’s intestines. Common points of attachment are the gills, fins, or skin of fish.
Endoderm —The tissue within the flatworm that lines its digestive cavity.
Free–living species —Nonparasitic; feeding outside of a host.
Hermaphrodite —Having the sex organs of both male and female.
Mesoderm —A spongy connective tissue where various organs are embedded.
Schistosomes —Blood flukes that infect an estimated 200 million people.
The New Larousse Encyclopedia of Animal Life. New York: Bonanza Books, 1987.
Pearse, John, et. al. Living Invertebrates. Palo Alto, CA: Blackwell Scientific Publications and Pacific Grove, The Boxwood Press, 1987.
University of California Museum of Paleontology. “Introduction to the Platyhelminthes: Life in Two Dimensions” <http://www.ucmp.berkeley.edu/platyhelminthes/platyhelminthes.html> (accessed November 24, 2006).
University of Heidelberg. “Marine Flatworms of the World” <http://www.rzuser.uniheidelberg.de/~bu6/flatintr.htm> (accessed November 24, 2006).
"Flatworms." The Gale Encyclopedia of Science. . Encyclopedia.com. (October 17, 2018). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/flatworms
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