i would not suggest to make the snake move head first. a snake moves sideways and a little bit forwards multiple times bending the body back and forth while doing that. so an idea for realistic snake would be to group the snake to itself, and bind the locator of the group to the path, and then only animate the bending of the body independently from the snake group. if you move the locator of the group to the bottom, then the snake wont penetrate the floor, if the path is layed out at floor.