Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
After Pete, Everett and Delmar fall prey to the singing of three sirens, Everett and Delmar wake up on the banks of a stream and find Pete's clothing spread out on the ground as if he evaporated and left nothing but his beating heart which they can see under the shirt. But the beating they saw was just a leopard frog which crawls out of the shirt. Delmar screams and says that them sirens did that to Pete - they loved him up and turned him into a horney toad. The frog hops into the river making Pacific Treefrog sounds. Delmar jumps in and catches him and talks to him. They drive away and after a long silence Everett says "I'm not sure that's Pete." Later at dinner Everett says Pete got what he deserved for fornicating with some whore of Babylon - it's obviously some kind of judgement on Pete's character - then he tries to order some nits or grubs from the waitress to feed the frog. Big Dan the evil bible salesman/Cyclops later throws the frog at a tree.