![]() lumbricoides is also very widespread, even in developed nations, although it is now a relatively uncommon problem in the United States. ![]() Ascaris lumbricoides is the largest nematode intestinal parasite found in humans females may reach lengths greater than 1 meter. Some are common intestinal parasites, and their eggs can sometimes be identified in feces or around the anus of infected individuals. These unsegmented worms have a full digestive system even when parasitic. Phylum Nematoda (the roundworms) is a diverse group containing more than 15,000 species, of which several are important human parasites (Figure 1). (credit: modification of work by Centers for Disease Control and Prevention) A micrograph of the nematode Enterobius vermicularis, also known as the pinworm. Some are monoecious, having both male and female reproductive organs in a single individual, while others are dioecious, each having either male or female reproductive organs. Parasitic forms may have complex reproductive cycles with several different life stages and more than one type of host. However, the parasitic species often have limited digestive tracts, nervous systems, and locomotor abilities. As animals, they are multicellular and have organ systems. Of the many species that exist in these groups, about half are parasitic and some are important human pathogens. There are two major groups of parasitic helminths: the roundworms (Nematoda) and flatworms (Platyhelminthes). Parasitic helminths are animals that are often included within the study of microbiology because many species of these worms are identified by their microscopic eggs and larvae.
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