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flatworm, likewise called platyhelminth, any kind of of the phylum Platyhelminthes, a team of soft-bodied, usually much flattened invertebrates. A variety of flatworm varieties are free-living, but about 80 percent of every flatworms room parasitic—i.e., living on or in one more organism and also securing nourishment indigenous it. They space bilaterally symmetrical (i.e., the right and left sides space similar) and also lack specialized respiratory, skeletal, and circulatory systems; no body cavity (coelom) is present. The body is not segmented; spongy connective tissue (mesenchyme) constitutes the so-called parenchyma and fills the an are between organs. Flatworms are generally hermaphroditic—functional reproductive organs of both sexes arising in one individual. Like other progressed multicellular animals, castle possess three embryonic layers—endoderm, mesoderm, and also ectoderm—and have a head region that contains focused sense organs and nervous tissue (brain). Many evidence, however, suggests that flatworms are very primitive contrasted with other invertebrates (such together the arthropods and annelids). Some contemporary evidence says that at least some flatworm types may be secondarily simplified from more facility ancestors.

The phylum is composed of four classes: Trematoda (flukes), Cestoda (tapeworms), Turbellaria (planarians), and Monogenea. It must be detailed that part authorities take into consideration Monogenea, which contains the stimulate Aspidogastrea, to it is in a subclass in ~ the course Trematoda. Members of every classes other than Turbellaria are parasitic during all or part of the life cycle. Many turbellarians are solely free-living forms. An ext than 20,000 flatworm types have to be described.

General features

Importance

Although some platyhelminths (flatworms) are free-living and nondestructive, plenty of other species (particularly the flukes and tapeworms) parasitize humans, domestic animals, or both. In Europe, Australia, and also North and also South America, tapeworm infestations the humans have actually been greatly reduced as a consequence of routine meat inspection. However where sanitation is poor and meat eaten undercooked, the incidence that tapeworm infestations is high. In the Baltic countries much of the population is infested v the broad tapeworm (Diphyllobothrium latum); in components of the southern United states a little proportion the the population may be infested v the dwarf tapeworm (Hymenolepis nana). In Europe and the United claims the beef tapeworm (Taenia saginata) is common because of the habit of eat undercooked steaks or other beef products.

Parasites in immature stages (larvae) can reason serious damages to the host. A larval stage of the gid parasite of lamb (Multiceps multiceps) generally lodges in the lamb brain. Fluid-filled hydatid cysts (i.e., sacs containing plenty of cells qualified of occurring into brand-new individuals) of Echinococcus may occur practically anywhere in the human body of sheep. In humans, hydatids the the liver, brain, or lung are frequently fatal. Infestation occurs just where civilization live in close association v dogs that have access to infested sheep for food.


Thirty-six or an ext fluke species have to be reported as helminth in humans. Endemic (local) centres of infection happen in basically all countries, but widespread infections occur in the much East, Africa, and also tropical America. Many species are ingested together cysts, referred to as metacercariae, in uncooked food—e.g., the lung fluke Paragonimus westermani uncovered in crayfish and crabs, the minister flukes Heterophyes heterophyes and Metagonimus yokogawai and also the liver fluke Opisthorchis sinensis in fish, and also the intestinal fluke Fasciolopsis buski on plants. Free-swimming larvae (called cercariae) the blood flukes pass through the human being skin directly. In humans these parasites and others listed below cause much misery and death. Manage of details flukes through the eradication of your mollusk hosts has been attempted but without much success.

Schistosomiasis (bilharziasis) is a significant human disease caused through three varieties of the genus Schistosoma, known jointly as blood flukes. Africa and also western Asia (e.g., Iran, Iraq) room endemic centres because that S. Haematobium; S. Mansoni also is discovered in this areas, and also in the West Indies and South America. In the much East, S. Japonicum is the important blood fluke.

Among domestic animals, the lamb liver fluke (Fasciola hepatica) may cause debilitating and fatal epidemics (liver rot) in sheep. These animals become infected by eating metacercariae encysted ~ above grass. Monogenea are common pests on fish in hatcheries and home aquariums.

Size range

Most turbellarians are much less than five millimetres (0.2 inch) long, and also many space microscopic in size. The largest of this course are the planarians, which might reach 0.5 metre (about 20 inches) in length. Trematodes are greatly between around one and also 10 millimetres (0.04 come 0.4 inch) long; members of some species, however, may prosper to numerous centimetres. The smallest cestodes are around one millimetre (0.04 inch) long, but members that a couple of species exceed 15 metres (50 feet) in length. The Monogenea range in length from 0.5 to 30 millimetres (0.02 come 1.2 inches). Aspidogastrea are from a few millimetres to 100 millimetres in length.

Distribution and also abundance

In general, free-living flatworms (the turbellarians) can occur wherever over there is moisture. Except for the temnocephalids, flatworms are cosmopolitan in distribution. They happen in both fresh water and also salt water and occasionally in moist terrestrial habitats, especially in tropical and subtropical regions. The temnocephalids, which space parasitic ~ above freshwater crustaceans, occur primarily in central and south America, Madagascar, new Zealand, Australia, and also islands that the southern Pacific.

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Some flatworm varieties occupy a really wide variety of habitats. Among the most cosmopolitan and most tolerant of various ecological problems is the turbellarian Gyratrix hermaphroditus, which occurs in fresh water in ~ elevations indigenous sea level to 2,000 metres (6,500 feet) and also in saltwater pools. Adult creates of helminth flatworms space confined nearly entirely to details vertebrate hosts; the larval forms, however, take place in vertebrates and in invertebrates, especially in mollusks, arthropods (e.g., crabs), and annelids (e.g., naval polychaetes). They space cosmopolitan in distribution, yet their incident is carefully related to that of the intermediate organize or hosts.