Tapeworms

Just read a great post over at The Loom about tapeworms. Parasites are nasty creatures and it's something that's both scary and interesting to learn more about. Something about living inside another living creature and crawl around makes me squirm. Here's a quote from that post about Monogeneans:

Some monogeneans give birth to offspring without releasing them from their bodies. Their offspring mature inside them and give birth as well. Like a hideous Russian doll, a monogenean may contain twenty generations of descendents inside its body!

How weird is that? How do they evolve when the offspring living inside itself like this? Carl Zimmer who writes on this blog is an author of some very interesting posts and books, for example Parasite Rex.