I'm looking for a short story I read in school.  The gist of it was
that a girl ate all the time, but always stayed thin.  At the end, it
was revealed that she ate a tapeworm (or a tapeworm's egg?) that lived
in her stomach and ate the food.  I'd like either a link to the story,
or a link to a book that contains it so I can buy it.  The story was
called 'Symbiosis'.  I believe it was written by a woman.

Hello.

The story that you're looking for is probably "Symbiosis: Class
Cestoda" by Nicola Barker. It's available in a collection of Barker's
short stories called "The Three Button Trick and Other Stories."

See this brief description on the web site of Barnes & Noble:

"In 'Symbiosis,' a formerly pudgy ex-girlfriend shows off her new
svelte shape and the reason for it: a tapeworm."
http://search.barnesandnoble.com/bookSearch/isbnInquiry.asp?ISBN=0880016779

Used copies of "The Three Button Trick and Other Stories" are
available via alibris:
http://www.alibris.com/search/search.cfm?qwork=6681735&ptit=The%20Three%20Button%20Trick%20and%20Other%20Stories&pauth=Barker%2C%20Nicola&pisbn=0880016779&pbest=3%2E90&pbestnew=16%2E10&pqty=24&pqtynew=1&matches=24&qsort=r

search strategy: san francisco state university library catalog,
stories and symbiosis
I hope this helps.

Perfect: just what I was looking for:-)