An Introduction to Zoology: Chapter 11

Class Cestoda:

The cestodes are probably most people’s least-favorite flatworms. Cestodes are highly specialized parasites that live inside their hosts’ digestive systems. They’re commonly known as “tapeworms.”

A tapeworm’s body is almost entirely devoted to reproduction. It can do this because, since it lives inside its host’s digestive system, it has no need of a digestive system of its own. A tapeworm’s body consists of numerous segments, the first of which is known as the scolex. The scolex contains hooks with which it attaches itself to its host’s intestinal wall. If the host animal has many tapeworms living in its intestine, their hooks can do a lot of damage, causing internal bleeding that may result in anemia and even death.

The remainder of a tapeworm’s body is known as the strobila, and it consists of numerous reproductive segments. Each reproductive segment is known as a proglottid. Each proglottid is devoted more or less entirely to production of gametes, and it produces simply prodigious numbers of spermatozoa and ova. Most tapeworm species are hermaphroditic, and a tapeworm is capable of fertilizing its own eggs. Any tapeworm in a person’s gut will be shedding many thousands of eggs per day.

The proglottids, despite their simplicity, do have muscles. Occasionally, a chain of proglottids will break free of the tapeworm and crawl out of the host’s body through the anus. I’ve been told that it’s a somewhat … exciting … experience to have one crawl out of your lower intestine and down your leg.


Like so many other parasites, tapeworms generally have complex life cycles and infect several different hosts. A well-known example is the Beef Tapeworm (Taenia saginata), which has cows as its intermediate host and humans as its definitive host.

Anatomy of a typical cestode.



An 8.8-meter tapeworm preserved at the Parasite Museum
in Meguro, Japan. It was taken from a human. According
to the museum, the victim got it from eating sushi.