[b_sharp]

"For this and other reasons, it is unlikely that there will ever be radically new body plans evolving out of established species. New body plans can only emerge from rather simple body plans."

That depends on how you define radical. The therapsids mammals descended from evolved from reptiles. Is the body plan difference between reptiles and mammals radical?

[Doctor Stochastic]

Sacks and tubes and combinations thereof seem to exhaust most of the possibilities.

[js1138]

Is the body plan difference between reptiles and mammals radical?

No, not the body plan. There are only a few genes separating scales from feathers or fur.

But trying to breed mammals from reptiles or vice-versa poses a problem. Mutations do not have a direction, or more specifically, 150 years of observation and analysis has never detected a direction. Trying to breed a specific new creature, or trying to reverse evolution by selective breeding will run into the improbability wall.

I think of it as the Buckaroo Banzai principle. Wherever you, or evolution goes, there it is, but if you try to specify a destination in advance, you will not get there.