It's really odd... They have a line that goes down their stomachs that is I guess (from how it was explained to me) where the skin of the fetus converges and closes up after the development of internal organs. It's present on all snakes but is normally covered up by their belly scales. There are a couple pictures and videos floating around on the Internet.
EDIT: Oops, duplicate post. Tried editing the original and messed up I guess