You will crawl on your belly and you will eat dust all the days of your life.
And I will put enmity between you and the woman,
and between your offspring and hers;
he will crush your head, and you will strike his heel.'
Genesis 3:14-19 (NIV)
Such is the power of sin that even forgiven sin can leave scars. There was an old man named Mel who used to show up at our college-aged youth group from time to time, and he once remarked that sin was like a board which had nails hammered into it. You could remove the nails, but the holes would still be there. In other words, you could be forgiven, but still have lasting effects from sinful behavior. I didn't like that analogy, because it seemed to make Christ's sacrifice on the cross for our sins less effective. However, I think that what Mel was trying to impress upon us was that one couldn't flirt with sin and expect to escape unscathed. Someone can be forgiven, but still have to live with the effects which sin can leave upon our lives. For example, if a person was a bank robber, they might sincerely come to faith in Christ while serving time for their crime, yet still have to serve their sentence in jail. Or, a person who murders someone might later come to Christ, yet he cannot give his victim's life back to him. So those 'holes' remain, although by God's grace even these can become used by God for His purposes.
Satan would be cursed above all the livestock and wild animals. It appears he possessed the body of the snake, for he was originally an angelic being. Snakes would henceforth be reduced to crawling on their bellies (thus the 'eat dust' part of the curse.)
I have read that snakes have within their bodies some remnants of limbs, possibly evidence that they once possessed some type of limbs. However, other animals have had similar vestigial 'limbs' which were later found to be structures which had functions of their own apart from locomotion, so we should be careful not to say more than that which the Scripture says about this matter, which is that a change took place as a result of God's curse which forced the snake to crawl upon its belly.
This seems to at least indicate that at one time, snakes did not do so.
There would also be enmity between Satan and Jesus, and between Satan's 'offspring' and that of the woman. While Satan would strike at the woman's offspring's heel, the 'seed' of the woman would crush Satan's head. This indicates a fatal blow. The enmity which is referred to is the enmity which exists between Satan and Jesus, who is the seed of the woman. In other words, there is a battle going on even to this day between good and evil. However, when Jesus died on the cross and then was resurrected, He triumphed over Satan and the battle was already won at that point.