Topics: Cestoda, Reproductive system, Flatworm Pages: 4 (1268 words) Published: June 3, 2014
Cestoda is the name given to a class of parasitic flatworms, of the phylum Platyhelminthes. Informally biologists refer to them as Cestodes. The most notorious species commonly are called tapeworms, for example Taenia solium, the "pork tapeworm". Cestodes are all parasitic and their life histories vary, but typically they live in the digestive tracts of vertebrates as adults, and often in the bodies of other species of animals as juveniles. Over a thousand species have been described, and all vertebrate species may be parasitised by at least one species of tapeworm. Humans are subject to parasitism by several species of tapeworms if they eat underprepared meat such as pork, beef, and fish, or if they live in, or eat food prepared in, conditions of poor hygiene . T. saginata, the beef tapeworm, can grow up to 20 m ; the largest species, the whale tapeworm Polygonoporus giganticus, can grow to over 30 m . On the other hand, species using small vertebrates as hosts tend to be small. For example, vole and lemming tapeworms are only in length, and those parasitizing shrews only . Tapeworm parasites of vertebrates have a long history: recognizable clusters of cestode eggs, one with a developing larva, have been discovered in fossil feces of a shark dating to the mid- to late Permian, some 270 mya. The worm's scolex attaches to the intestine of the definitive host. In some species, the scolex is dominated by bothria, or "sucking grooves" that function like suction cups. Other species have hooks and suckers that aid in attachment. Cyclophyllid cestodes can be identified by the presence of four suckers on their scolex. While the scolex is often the most distinctive part of an adult tapeworm, it is often unnoticed in a clinical setting as it is inside the patient. Thus, identifying eggs and proglottids in feces is important. The main nerve centre of a cestode is a cerebral ganglion in its scolex. Motor and sensory innervation depends on the number and...
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