6.3.4: Cestodes (Tapeworms)

Cestodes are parasitic platyhelminths that have segmented ribbon-like bodies, they range in length from 1 mm to more than 15 m. They are commonly referred to as tapeworms. Like trematodes, they exhibit bilateral symmetry and are monoecious. Cestodes completely lack digestive systems for acquiring nutrients. Instead of ingesting nutrients, they absorb their nutrients across their cuticles. They also lack circulatory systems.

The typical adult cestode will have a head region, known as a scolex, a neck, and a body region that is referred to as a strobila. The scolex of a tapeworm is used for attachment to the host.

Thus, structures such as suckers and hooks are present. The structures present on the scolex for attachment vary fro one species to another and thus can aid in their differentiation. The neck region, contain cells that undergo mitotic divisions to produce new segments for the body portion of the parasites. The body or strobila, is composed of segments known as proglottids. Starting at the proglottid closets to the neck, proglottids are referred to as immature. These are newly formed proglottids and reproductive structures have not yet developed within the segment. As proglottids move further away from the neck they develop male and female reproductive structures; testes, ovaries, uterus, etc. Each proglottid will have both male and female sex organs and thus each segment is in itself monoecious. These proglottids can then undergo cross-fertilization from another individual) or self-fertilization. Cross-fertilization is more common. However, self-fertilization offers the parasite the ability to continue to reproduce even in the absence of a separate individual within their environment. Fertilization will result in the production of eggs. Proglottids containing eggs are known as gravid proglottids.

The adult cestode resides within its definitive host. Gravid proglottids and eggs are shed in fecal material from the host. The typical life cycle then resumes as an intermediate host ingests the eggs or gravid proglottids which develop into larval forms within the intermediate host. Cestodes may have one or more intermediate hosts. Hymenolepis nana is an exception to this typical life cycle and can have a single host within its life cycle.

In this exercise we will survey six of the many parasitic cestodes that exist.

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