The phyla Nematoda mostly consists of a kind of worm named roundworms. Nematodes are bilaterally symmetrical. Reproduction is commonly sexual, but asexual reproduction can happen. Nematodes body plan consists of digestive system, and excretory system, and a nervous system. An alimentary canal is a canal that travels all the way from the mouth to the anus. Nematodes don't have a circulatory system, but they use diffusion to transfer nutrients. Parasitic roundworms in the genus Trichinella cause Trichinosis.  People get this disease through eating undercooked meat and can go without symptoms. I find it interesting that a worm can be parasitic and cause a disease.

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Eelworm

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Threadworms

Threadworms are small white parasitic worms that infect the intestines and attack your internal organs.

Pinworm

The pinworm is another parasitic nematode that infect the intestines. The medical disease caused by pinworms is called Enterobiasis.