Editing Flatworm (section)

====Cestoda====

{{main|Cestoda}}

[[File:Taenia solium Life cycle.tif|thumb|300px |Life cycle of the eucestode ''[[Taenia (flatworm)|Taenia]]'': Inset 5 shows the scolex, which has four ''[[Taenia solium]]'', a disk with hooks on the end. Inset 6 shows the tapeworm's whole body, in which the scolex is the tiny, round tip in the top left corner, and a mature proglottid has just detached.]]

These are often called tapeworms because of their flat, slender but very long bodies – the name "[[cestoda|cestode]]" is derived from the [[Latin]] word ''cestus'', which means "tape". The adults of all 3,400 cestode species are internal parasites. Cestodes have no mouths or guts, and the [[syncitium|syncitial]] skin absorbs nutrients – mainly [[carbohydrate]]s and [[amino acid]]s – from the host, and also disguises it chemically to avoid attacks by the host's [[immune system]].<ref name="RuppertBarnes2004Platyhelminthes" /> Shortage of carbohydrates in the host's diet stunts the growth of parasites and may even kill them. Their [[metabolism]]s generally use simple but inefficient chemical processes, compensating for this inefficiency by consuming large amounts of food relative to their physical size.<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" />

In the majority of species, known as eucestodes ("true tapeworms"), the neck produces a chain of segments called proglottids via a process known as [[strobilation]]. As a result, the most mature proglottids are furthest from the scolex. Adults of ''[[Taenia saginata]]'', which infests humans, can form proglottid chains over {{convert|20|m|ft}} long, although {{convert|4|m|ft}} is more typical. Each proglottid has both male and female reproductive organs. If the host's gut contains two or more adults of the same cestode species they generally fertilize each other, however, proglottids of the same worm can fertilize each other and even themselves. When the eggs are fully developed, the proglottids separate and are excreted by the host. The eucestode life cycle is less complex than that of [[digenea]]ns, but varies depending on the species. For example:
* Adults of ''[[Diphyllobothrium]]'' infest fish, and the juveniles use [[copepod]] crustaceans as intermediate hosts. Excreted proglottids release their eggs into the water where the eggs hatch into [[cilia]]ted, swimming larvae. If a larva is swallowed by a copepod, it sheds the cilia and the skin becomes a syncitium; the larva then makes its way into the copepod's [[hemocoel]] (an internal cavity which is the central part of the [[circulatory system]]) where it attaches itself using three small hooks. If the copepod is eaten by a fish, the larva [[metamorphosis|metamorphoses]] into a small, unsegmented tapeworm, drills through to the gut and grows into an adult.<ref name="RuppertBarnes2004Platyhelminthes" />
* Various species of ''[[Taenia (flatworm)|Taenia]]'' infest the guts of humans, cats and dogs. The juveniles use herbivores – such as pigs, cattle and rabbits – as intermediate hosts. Excreted proglottids release eggs that stick to grass leaves and hatch after being swallowed by a herbivore. The larva then makes its way to the herbivore's muscle tissue, where it metamorphoses into an oval worm about {{convert|10|mm|in}} long, with a scolex that is kept internally. When the definitive host eats infested raw or undercooked meat from an intermediate host, the worm's scolex pops out and attaches itself to the gut, when the adult tapeworm develops.<ref name="RuppertBarnes2004Platyhelminthes" />

Members of the smaller group known as [[Cestodaria]] have no scolex, do not produce proglottids, and have body shapes similar to those of diageneans. Cestodarians parasitize fish and turtles.<ref name="WalkerAnderson2001PlatyhelminthesInAnderson" />
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