Animals
HookwormsHookworms, necator americanus, are invertebrate with slender bodies that are bilaterally symmetric. Nematodes in general do not have an actual circulatory system, and they are able to transport their nutrients and oxygen by diffusion.
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Trichina Worm
The trichinella sprialis, trichina worm, is a nematode that also has no circulatory system. It uses diffusion to move oxygen and nutrients through its body.
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Loa LoaLoa loa is commonly known as the "eye worm," and it is the filarial nematode that causes Loa loa filariasis. Like all other nematodes, loa loa has no actual circulatory system and absorbs its nutrients from liquids under the skin and eyeball and diffuses it throughout its body.
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Nematoda
Nematoda is an animal phylum of 25,000 species and half of those species are similar to parasites. Nematodes do not live in dry areas because they have no circulatory systems. Therefore, they take in their nutrients from directly from their environment and use diffusion to transport oxygen and those nutrients throughout the body.