Jam-first songs?

Do any Phish songs open with an extended jam or improvisation, and then segue into a more composed section? Rather than the composed section --> freewheeling jam structure.

This is not a great example, but I've heard some versions of David Bowie open with variations of dissonant/creepy ambient stuff or looping arpeggios before they hit the "ba-da-ba-ba! ba-ba!" riff.