Nematodes and platyhelminthes are more commonly referred to generically as roundworms and flatworms (or tapeworms). There are over 16,000 parasitic species of nematodes, while over half of the 12,000-plus platyhelminthes species are also parasitic. The major differences between the nematodes and platyhelminthes are their shapes (round versus flat) and how they process what they ingest. Roundworms have an actual digestive system, while flatworms only have a generalised digestive gut.
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Nematode digestive systems are like those of many worm species. They have an opening in one end through which nutrients pass and an opening at the other end where wastes are expelled. They are also slender and are usually under 1/10 inch in length. Many are so small, you'll need a microscope to see them; others can be more than 2 inches long. Nematode heads are large and distinctive.
Platyhelminthes, unlike nematodes, don't actually have a digestive system. Rather, they take in nutrients and oxygen through their flattened bodies and then regurgitate wastes through their mouth organs, though some do have anuses. Most don't have specialised respiratory or circulatory systems, also unlike nematodes. In addition, they're bilateral, meaning the have identical right and left sides, whereas nematodes are rounded, with no particular side.
Nematodes come as both males and females and reproduce sexually, whereas platyhelminthes tend to be hermaphoditic. This means they have the ability to reproduce themselves without benefit of sex with other platyhelminthes. While roundworms tend to have widespread nervous systems, all flatworm species have systems which are concentrated around their head ends. Platyhelminthes tend to concentrate in watery or moist environments, while nematodes can be found inhabiting every environment, dry or wet, in the world.
Both nematodes and platyhelminthes are parasites of humans and can cause a number of serious illnesses. In severe parasitic attacks, both can even cause death if they infect their hosts in sufficient numbers. Common nematode infections include those of hookworms and pinworms as well as the trichina, which can cause trichinosis. Humans eating raw or inadequately cooked pork products can contract the worm through that route. Schistosome platyhelminthes are known for causing potentially lethal schistosomatic fevers.
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