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6.2: Introduction

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    Helminths are a group of worms most often referred to as the parasitic worms—as opposed to earthworms, polychaete worms, etc. Worms are multicellular, eukaryotic, invertebrate animals. Most are macroscopic, but have microscopic life stages. They are complex organisms with organs and organ systems. In addition, they usually have complex life-cycles which may include different larval and cyst stages, and multiple hosts.

    Terms to know

    Intermediate host: the host in which larval development occurs

    Definitive host: the host in which mature organisms live and undergo sexual reproduction

    Hermaphroditic: male and female sex organs occur in the same individual worm (most commonly in cestodes and many trematodes).

    Parasite: an organism that lives in or on another organism (the host), to the detriment of the host.

    Taxonomy: The taxonomy of organisms is under constant review. We will use the following taxonomy to group the helminthes we will observe.

    Phylum Platyhelminthes (flatworms)

    Phylum Nematoda (roundworms)

    Class Trematoda

    Class Cestoda


    -Leaf shaped

    -Mollusk is usually the intermediate host(s)

    -Vertebrate is usually the definitive host

    -Digestive system


    Adult of C. sinensis stained with carmine. Clearly visible in this image are the oral sucker (OS), pharynx (PH), ceca (CE), acetabulum, or ventral sucker (AC), uterus (UT), vitellaria (VT) and testes (TE).


    -Long, ribbon like bodies

    -Scolex for attachment, with reproductive segments called proglottids

    -Usually also require two or more hosts

    -No digestive system


    Scolex of T. solium. Note the four large
    suckers and rostellum containing two
    rows of hooks.



    -Usually only one host

    -Complete digestive system

    -Very diverse and numerous group.

    -Many free-living, predatory, and parasitic types.


    Adult Ascaris lumbricoides

    Contributors and Attributions

    This page titled 6.2: Introduction is shared under a CC BY license and was authored, remixed, and/or curated by Kelly C. Burke.

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