When you put a slug or snail on a sheet of glass and look at it's muscular foot from beneath, you see waves of dark and light bands moving from the back of the slug to its front.  I understand that these are muscular contractions that move in waves to propel the slug, but if they are moving from the rear of the slug to the front, shouldn't the slug be propelled backwards (ie - in the opposite direction to the motive force)?