There are 14 000 species of “flat worms”; most of it are parasits of animals, including of the human kind. Parasit flaworm’s developpement can necessitate up to 4 intermediate hosts of different species before the worm is ready to "settle" in a definitive host (wich are usually mammals). The tapeworm, a flatworm who’s definitive host can be the human, can grow up to 20 meters long.
The bloodsucker/leech’s size ranges from 1 to 30 cm (when starved). When it eats, it sucks as much blood as it can contain, wich is up to ten times its own mass. After that it won’t eat again for several months. Leech's saliva contains anesthetic and anticoagulant so that the host notices nothing while it's feeding. You can easily get rid of a bloodsucker stuck after you by putting salt on it and around the “bite”. <img src="/ubbthreads/images/graemlins/winkwink.gif" alt="" />
Darwin evaluated that, in England, there are about 125 000 earthworms by hectare; those 125 000 worms produce approximately 45 tons of excrement per year. Earthworms have five “hearts” and are hermaphrodite. Their size goes from 1 mm to 3 m (Megascolides australis).
