A rather fascinating page on parasites.

ETA HURRRR, I posted the 1,337th entry!

Flatworms have no body cavity other than the gut (and the smallest free-living forms may even lack that!) and lack an anus; the same hole both takes in food and expels waste. Because they don't have any other cavity they have to be flat, or parts of them will suffocate or starve - there's simply nothing to make a circulatory system out of ( Read more... )