NEMATODES (THE ROUNDWORMS) These worms are round in cross-section and tapered at each end. They have a complete digestive system, with a mouth, an intestine, and an anus. Most are dioecious. Males are smaller than females. Many nematodes are free-living in soil and water, but there are also many parasites of plants and animals. Nematode parasites have simpler life cycles than the flatworms. Nematode infections in humans can be divided into 2 categories: 1. Eggs infective for humans a. Enterobius vermicularis —the pinworm---this parasite spends its entire life in the large intestine of a human host. The female pinworm migrates to the anus to leave eggs on the perianal skin while the host sleeps. The eggs can then be ingested by the host or another person exposed through contaminated clothing or bedding. b. Ascaris lumbricoides
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