Build Me A Tapeworm

Darwin gave a lot of thought to the strangest creatures on this planet, wondering how they had evolved from less strange ancestors. Whales today might be fish-like warm-blooded beasts with blowholes and flukes, but long ago, Darwin argued, their ancestors were ordinary mammals that walked on land with legs. His suggestion was greeted with shock and disbelief; neverthless, scientists have found bones from ancient walking whales. Humans, Darwin argued, evolved from apes, most likely in Africa where chimpanzees and gorillas are found today. And today scientists have found about twenty different species of hominids, from chimp-like creatures that lived six million years ago to not-quite humans that lived alongside our own species. Darwin also pondered the origins of barnacles, orchids, and many other strange creatures. But for some reason--perhaps thanks to his famously weak stomach--Darwin didn't write a single word about tapeworms. It's a pity, because tapeworms are as strange as animals can get...

These flat, ribbon-like creatures live inside the digestive tracts of vertebrates. The tapeworms that live in humans can get up to sixty feet long. They feed on our food, despite the fact that they have neither a mouth nor a digestive tract. Their bodies are like a kind of inside-out intestine, rippling with finger-like projections that absorb nutrients. Once inside us, tapeworms can live for decades, deftly escaping the notice of the immune system despite their being as long as an anaconda. Some tapeworms have hooks or suckers on their front end ("head" is too generous a term), which they use to anchor themselves in place. They can also swim upstream to meet food coming out of the stomach and drift back down the intestines to feed, releasing chemicals to slow down their host's peristalsis so that they don't get swept away.