In the Loveless manga, how did Soubi lose his ears?

Or didn't they say yet? I can't remember. When I was reading the manga I think I umed Ritsu (? was that his name? anyway Soubi's teacher guy) d him because he was so cruel to poor Soubi, but I wasn't sure... It obviously wasn't Seimei because he still has it ears... unless he has fake ears like Kouya (? not sure about this name either lol). What do you guys think?