A tapeworm starts off as an egg which then develops into a cyst. Inside the cyst is a ball-shaped creature with hooks that it can use to crawl around its host before growing into an adult. Many species are made up of dozens or hundreds of segments called proglottids. Each proglottid may be equipped with both eggs and sperm-making organs. As an adult, a tapeworm also grows a head-like end often equipped with suckers or hooks of its own. This strange organ is called the scolex. (The shark tapeworm in this photo is displaying its fearsome scolex.) While the tapeworm lives in the gut of its host, it uses its scolex to clamp down in place, although it may swim around to find an ideal spot from time to time.
And from time to time, a proglottid breaks off from the body of the tapeworm and ends up leaving the body. Actually, it can leave under its own steam. Proglottids have muscles and nerves that they can use to crawl out the back end and along the ground.
Tapeworms evolved from free-living flatworms, but they have undergone some radical changes. Along with the evolution of their proglottids, they also abandoned their digestive tract, opting instead to slurp up their food directly through their skin. They’re so weird now that scientists haven’t even been sure which end is which. Some people have suggested the scolex is the head of the tapeworm. But others have pointed out that while the tapeworm is still in its cyst, the hooks actually form on the other end of its body. What’s more, in related flatworms with recognizable heads and tails, the sperm-organs are closer to the head than the ovaries. In tapeworms, the ovaries are closer to the scolex.
This morning at the second day of the American Society of Parasitologists, Peter Olson from the Natural History Museum in London, offered a potential solution to the puzzle. All animals–including us–use a set of master genes to determine the head-to-tail anatomy of developing embryos. The precise DNA sequence of these genes is different from species to species, but they show clear evidence of having evolved from a set of genes in a distant ancestor. Scientists have carried out most of their research on these genes in well-studied species like fruit flies and mice. Only recently have scientists started to look at how these so-called Hox genes work in other animals. Olson is studying the genes in tapeworms that live in mice, called Hymenolepis.
One key gene Olson described is called Post-2. It corresponds to genes that defines the tail end of insects and mammals. When tapeworms develop into little balls with hooks, Olson has discovered, Post-2 becomes active on the end of the ball with the hooks. That suggests that the hooks are growing at the tail-end of the animal, before it has yet grown a tail.
Later, when the tapeworm develops into an adult with proglottids, Post-2 becomes active at one end of each segment. It becomes active at the end furthest from the scolex. So the tail end of the animal appears to be pointing away from the scolex–in other words, the scolex really is the head after all. It may not be a head you’re familiar with–sans teeth, sans eyes, sans taste, although not quite sans everything. But when the tapeworm grew its head, it knew where it was the same way you knew when you grew yours.
[Edited a bit to address questions from commenters.]