How does the tapeworm body plan support widespread dissemination of the parasite?

The cestodes are internal parasites, mainly of vertebrates. Tapeworms, such as those of Taenia live in the intestinal tract of their primary host and remain fixed using a sucker or hooks on the anterior end, or scolex, of the tapeworm body, which is basically a colony of similar subunits called proglottids. Each proglottid could contain an excretory system with flame cells, along with reproductive structures, of both sexes. They are so long and flat that they tapeworms do not need a digestive system; they absorb nutrients from the food matter surrounding them in the host’s intestine by diffusion.

Proglottids are produced at the scolex and eventually migrate to the end of the tapeworm; at this point, they are “mature” and all structures except fertilized eggs have degenerated. Maximum reproduction occurs by cross-fertilization between different worms in the same host, but it may also occur between proglottids. The mature proglottids that detach from the body of the worm and are released into the feces of the organism. The eggs are then eaten by an intermediate host, typically another vertebrate. The juvenile worm infects the intermediate host and takes up residence, usually in muscle tissue. Once the muscle tissue is consumed by the primary host, the cycle is completed. There are several tapeworm parasites of humans that are transmitted by eating poorly cooked pork, beef, or fish.