(Roundworms)
The Nematodes are perhaps the secound largest group of organisms behind the arthropods. They are generally described as nonsegmented roundworms and are commonly found in the soil. They also inhabit fresh and salt water, and other more specialized habitats.
They are a diverse group, and it should be noted that most members of the group are not parasitic. With that being said, the parasitic forms have developed highly specific biological dependencies on particular hosts. Without theses hosts the parasite would not be able to survive. There are also some members that have obtained the ability to adapt to a variety of hosts. There are a number of Nematodes that are clinically significant to man.
They have a number of rather complex anatomical and physiological characteristics. Some important characteristics are the following:
- The name roundworm originates from the fact that when sliced in a cross section fashion the worm will have a round shape to it.
- Movement is through the use of a muscular system that lies just underneath a protective layer called the cuticle.
- The cuticle is resistant to chemicals and each Nematode species has it's own unique structure and composition.
- The digestive tract of Nematodes is divided into three main parts. An oral (buccal) cavity/esophagus; midgut; and hindgut/anus. The oral and hindgut are lined with epithelial cells extending from the cuticle while the midgut is lined with columnar cells lined with microvilli. The columnar cells in the midgut area allow for the absorption of ingested nutrients while the muscular esophagus functions in the delivery of food to the midgut.
- Females are larger then the males.
The main Nematodes that are clinically significant to man are the following:
- Ascaris lumbricoides or "giant roundworm"
- Ancylostoma duodenale or "Old World Hookworm"
- Baylisascaris spp.
- Brugia malayi
- Brugia timori
- Dipetalonema perstans
- Dipetalonema streptocerca
- Enterobius vermicularis or "pinworm"
- Gnathostoma spp.
- Loa loa
- Mansonella ozzardi
- Necator americanus or "New World Hookworm"
- Onchocerca volvulus
- Strongyloides stercoralis or "Thread Worm"
- Toxocara canis
- Trichinella spiralis or "Trichina Worm"
- Trichostrongylus spp.
- Trichuris trichiura or "whipworm"
- Wuchereria bancrofti
- Dracunculus medinensis or Guinea Worm
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