Nature's Perfect Shape

Another riddle in a poem. Should be an easy guess.

With nature's most efficient shape, begin.
Mix all the colors from His bow within.
Leave room enough to hold a symphony,
Or for an eardrum splitting cacaphony.
Enfolded safe within their fortress wall
Are things that walk and swim and fly and crawl.
In time, they gnaw the wall or find a flaw
And breakout to the world of fang and claw.