Same story, sounds like, in Discover (June 98):
Mighty Hermaphroditey
"Some hermaphroditic flatworms mate hypodermically--that
is, a flatworm must jab its penis into its partner and inject
sperm. The sperm courses through the loose tissue of a
flatworm's body until it reaches the ovaries. This mating
method is not uncommon in flatworms, but researchers
didn't realize that mating could escalate into a violent contest."
http://www.discover.com/june_story/break.html
(A fun experiment I saw on TV around Easter: take two marshmallow
chicks, insert a toothpic in each chick chest about 1/4 inch, put the
chicks about 1 inch apart in the microwave facing each other. Turn on
the microwave for approx 1 minute and the chicks will "joust". Depending
on how drunk you are, you can go through 2 or 3 boxes of chicks before
the novelty wears off.)