Agile Vs Spiral Model for SDLC

Question

I believe that Agile is nothing but another implementation of Spiral Model. I am a big supporter of Spiral (The spiral model is a software development process combining elements of both design and prototyping-in-stages, in an effort to combine advantages of top-down and bottom-up concepts) since its beginnings and have seen that lot of projects implement Spiral without knowing that they are operating in a Spiral world. Since the day Agile started gaining popularity the concept of spiral started getting overlooked a little bit. I am sure that for complex projects spiral is still the best alternative but I would like to get a better understanding of the similarities and differences between Agile and Spiral techniques. Can anyone explain their differences/similarities?