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Platyhelminthes

Platyhelminthes (plăt´ēhĕlmĬn´thēz), phylum containing about 20,000 species of soft-bodied, bilaterally symmetrical, invertebrate animals, commonly called flatworms. There are four classes: the free-living, primarily aquatic class, Turbellaria, and Trematoda, Cestoda, and Monogenea, which are considerably modified for their exclusively parasitic existence. While the structure of the flatworms marks a major step in animal evolution, their origin and relationships within the group are still controversial.

Anatomy

Flatworms are dorso-ventrally flattened. The epidermis is generally ciliated in the turbellarians, while trematodes and cestodes are covered with a cuticle. Beneath the outer covering are two layers of muscle, an outer circular layer, and an inner longitudinal layer; this arrangement permits an undulating form of locomotion that can be observed in the larger turbellarian species. A saclike digestive cavity, with a single opening to the outside that serves as both mouth and anus, is sometimes present; in the simpler forms it is absent or unbranched, but in higher forms it branches to all parts of the body. The major sense organs, when present, are concentrated in the head, or front end. Although a primitive nerve net is present in some of the simpler forms, others have several nerve cords extending from a brain along the length of the body. The latter pattern of organization is retained in the nervous systems of higher invertebrates, specifically annelids and arthropods.

The reproductive system of flatworms is characteristically hermaphroditic (i.e., each individual produces both eggs and sperm), and cross-fertilization between individuals is typical. While trematodes and cestodes shed eggs almost continuously, turbellarians exhibit seasonal reproductive activity and, in addition, display asexual reproduction and the ability to regenerate severed parts of the body.

All except the simplest flatworms have nephridial tubules, called protonephridia, usually distributed throughout the body. Such structures consist of an external opening and a tubule that branches internally, terminating in a number of blind, bulb-shaped structures called flame bulbs, which bear tufts of cilia. They probably function as excretory and osmoregulatory organs.

Class Turbellaria

The mostly free-living, primarily carnivorous, flatworms of class Turbellaria are characterized by a soft epidermis that is ciliated, at least on the ventral surface. The movement of the cilia propels the smaller forms. Larger species glide along by muscular waves, usually over mucous beds secreted by special cells.

Turbellarians are generally divided into five groups based primarily on differences in the form of the digestive cavity, a structure that is readily observable through the transparent body wall. The most primitive turbellarians, the acoels, have no digestive cavity. The ventral mouth, and sometimes a simple pharynx, lead to an inner mass of nutritive cells. Most species measure less than 1/8 in. (3 mm) in length.

The rhabdocoels have straight, unbranched digestive cavities. Some authorities believe that the rhabdocoels gave rise to both the trematodes and cestodes because several rhabdocoel species exhibit commensal relationships, which presumably could have given rise to parasitism. The allocoels were formerly classified together with the rhabdocoels; the gut can be either saclike or branched.

The triclads, also known as planarians, are relatively large flatworms named for their three-branched gut. Most species range from 1/8 in. (3 mm) to about 1 in. (2.5 cm) in length. Planarians have more sense organs and a more complex brain than the other turbellarians. The freshwater species Dugesia tigrina has primitive eyes and tactile lobes, or auricles, on the sides of the head. The muscular pharynx can be extruded for food capture. Dugesia and many other planarians can regenerate entirely new individuals from small pieces cut from the body.

The group of turbellarians known as polyclads tend to be larger (1–2 in./2.5–5 cm) and more oval-shaped than the triclads. Their bodies are extremely flat and leaflike, and the gut is subdivided into numerous branches. Many are brightly colored and some have ruffled edges. Some species have numerous eyes scattered over the front end of the body.

Class Trematoda

The parasitic flatworms of class Trematoda, also called flukes, have oral suckers, sometimes supplemented by hooks, with which they attach to their vertebrate hosts. Trematodes have retained the same body form and digestive cavity as the turbellarians. However, practically the entire interior is occupied by the reproductive system; the organism is capable of producing huge numbers of offspring. Trematodes of the order Digenea have complex life cycles involving two or more hosts. The larval worms occupy small animals, typically snails and fish, and the adult worms are internal parasites of vertebrates. Many species, such as the liver fluke Clonorchis sinensis and the blood fluke (Schistosoma), cause serious diseases in humans.

Class Cestoda

The body of the cestodes, also known as tapeworms, has lost the typical turbellarian form. Although there are a few unsegmented species, the bulk of a typical cestode body consists of a series of linearly arranged reproductive segments called proglottids. There is no mouth or digestive system; food is absorbed through the cuticle. Adults live in the digestive tract of vertebrates, and larval forms encyst in the flesh of various vertebrates and invertebrates.

The body of an adult tapeworm is virtually a reproductive factory. Behind a small securing knob, called a scolex, which bears a circle of hooks or other attachment organs, the proglottids constantly bud off and gradually enlarge. As they mature they become filled with male and female reproductive organs. Cross-fertilization takes place with adjacent worms or neighboring proglottids; in some cases self-fertilization occurs. In some species the ripe proglottids, filled with eggs, are shed. In others the fertilized eggs leave the adult host in the feces. If the eggs are consumed by the intermediate host, the life cycle continues. Tapeworm species that infest human intestines as adults include Taenia saginata, T. solium, the dwarf tapeworm, Hymenolepsis nana, and the fish tapeworm, Diphyllobothrium latum, which can reach lengths of up to 50 ft (15 m).

Class Monogenea

Monogenetic flukes spend their entire life cycle as parasites on a single host, often on the gills and skin of fish; they include no human parasites. They hold on to the fish by the use of hooks and attachment organs at the posterior end. Most of the parasite's body space is devoted to the hermaphroditic reproductive system. The egg on hatching releases a ciliated larva that enables the parasite to reach a new host. Species of the genus Gyrodactylus can can be serious pests in hatcheries, particularly since a single worm can give rise to more than one hundred descendants in three weeks.

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Platyhelminthes

Platyhelminthes

The phylum name Platyhelminthes literally means "flatworms." Members of this phylum are soft, thin-bodied, leaf or ribbonlike worms, including the familiar planaria of ponds and streams, as well as the flukes and tapeworms parasitic in human and other animal bodies. Some defining characteristics of the phylum are that flatworms are acoelomate (they have no body cavity), triploblastic (the body has three tissue layers), and bilaterally symmetric (they have symmetric right and left sides and usually a definite head), and they have organ systems, including an excretory, digestive, reproductive, and nervous system, but no respiratory system.

The class Turbellaria includes all free-living members of the phylum, as well as a few parasites . It includes many marine forms, whose beautiful colors serve as a warning of their toxicity to would-be predators, as well as the more drab freshwater planarians (Dugesia ). Some Turbellaria can swim by undulations of the body margins, but most of them glide gracefully over surfaces along a trail of mucus, pushed by cilia on their ventral surface.

The class Trematoda, commonly called flukes, are unsegmented parasitic flatworms that usually parasitize a snail as an intermediate host (in which they reproduce asexually) and a human or other vertebrate as a definitive host (in which the worms mate and lay eggs). Many species have other hosts between these two, such as fish or frogs. Trematodes usually have a pair of suckers for crawling and clinging to the host's tissues. Many humans are infected with blood flukes, liver flukes, lung flukes, and other trematode parasites of great medical importance.


HYMAN, LIBBIE HENRIETTA (18881969)

U.S. zoologist famous for her authoritative six-volume treatise on invertebrates, whose own specialty was hydras and flatworms. Hyman went to college despite her family's objections. In her last years, she lived off the royalties from her textbooks and worked at the American Museum of Natural History.


The Cestoda, commonly called tapeworms, are segmented, ribbonlike parasites usually found as adults in the small intestines of vertebrate animals. Unlike the other classes, they have no digestive tract, for they can absorb predigested nutrients from the host's intestine. The body consists of a long chain of segments, each with its own reproductive system. The anterior end is a knoblike holdfast called a scolex, equipped with suckers and often hooks for attachment to the host's intestine. In general, tapeworm infections are not as medically serious as trematode infections, but some tapeworms can be lethal.

see also Animalia; Body Cavities; Nematode; Parasitic Diseases; Symbiosis

Kenneth S. Saladin

Bibliography

Pechenik, Jan A. Biology of the Invertebrates, 4th ed. Boston: McGraw-Hill, 2000.

Schmidt, Gerald D., and Larry S. Roberts. Foundations of Parasitology, 6th ed. Dubuque, IA: McGraw-Hill Higher Education, 2000.

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Platyhelminthes

Platyhelminthes

Animals in the phylum Platyhelminthes are called flatworms because they are flattened from head to tail. Flatworms share several features with more derived animal phyla. They are the most primitive group to exhibit bilateral symmetry . Flatworms have three embryonic tissue layers: ectoderm, mesoderm , and endoderm.

Animals within Platyhelminthes show more complexity than ancestral phyla, but are not as complex as more derived animal phyla. They are acoelomates, which means they do not have a body cavity. Platyhelminthes are unsegmented. They have muscles and a simple nervous system that includes a primitive brainlike structure which is formed from a thickening of the ventral nerve cords in the head region. They have a mouth, but no anus, and a primitive digestive cavity. They also have a primitive excretory system. They do not have a respiratory or circulatory system and are limited to simple diffusion for gas exchange. They can regenerate by fission as well as reproduce sexually, sometimes with complex life cycles passing through more than one host. Flatworms move about using cilia and by undulating movements of the whole body.

Almost all Platyhelminthes are aquatic, both fresh water and marine, but a few terrestrial species live in moist, warm areas. Species vary in size from microscopic to over 60 feet (20 meters) long for some tapeworms.

There are four major classes of Platyhelminthes and over twenty-five thousand species. Flatworms in the class Turbellaria are marine and freshwater free-living scavengers . The other three classes are parasitic and include some of the most harmful human parasites. The classes Trematoda, commonly called flukes, and Momogea are both endoparasites and ectoparasites . Momogea are parasites of aquatic vertebrates such as fish. Flatworms in the class Cestoda are endoparasites known as tapeworms.

see also Phylogenetic Relationships of Major Groups

Laura A. Higgins

Bibliography

Anderson, D. T., ed. Invertebrate Zoology. Oxford: Oxford University Press, 1998.

Barnes, Robert D. Invertebrate Zoology, 5th ed. New York: Saunders College Publishing, 1987.

Campbell, Neil A., Jane B. Reece, and Lawrence G. Mitchell. Biology, 5th ed. Menlo Park, CA: Addison Wesley Longman, Inc., 1999.

Purves, William K., Gordon H. Orians, H. Craig Heller, and David Sadava. Life the Science of Biology, 5th ed. Sunderland, MA: Sinauer Associates Inc. Publishers, 1998.

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Platyhelminthes

Platyhelminthes (flatworms) A phylum of acoelomate, triploblastic, dorso-ventrally flattened (hence their name), bilaterally symmetrical worms in which the internal organs are well developed and metameric segmentation is absent. The gut may be present, but there is no anus and no blood-vascular system; protonephridia (see NEPHRIDIUM) are the excretory and osmoregulatory organs. Skeletal elements are rare. Morphologically they are quite diverse, and many are parasites. Most are hermaphrodites. The phylum lacks a definitive fossil record. From various evolutionary considerations it is probable that platyhelminths were in existence 1000–800 Ma ago.

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Platyhelminthes

Platyhelminthes A phylum of acoelomate invertebrates comprising the flatworms, characterized by a flattened unsegmented body. The simple nervous system shows some concentration of cells at the head end. The mouth leads to a simple branched gut without an anus. Flatworms are hermaphrodite but self-fertilization is unusual. Many species are parasitic. The phylum contains the classes Turbellaria (planarians), Trematoda (flukes), and Cestoda (tapeworms).

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