"Ha! Ha! Said the Clown"

In "Ha! Ha! Said the Clown," there's a particular sound under "this" in the line "In a while, run a mile, I'm regrettin' all this."  I'm pretty sure this is a Mellotron cello.  It seems that Mann pressed a key so that the note started, and then he turned the pitch knob so that the note goes up in pitch.  Whatever it is, this glissando-type effect illustrates the entirety of the "all" in "all this."