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.