The greatest barrier to many-celled organisms with wheels is the interface (meeting point) between the parts that move and the parts that stay still. Wheels, or wheels and axles, need to be able to turn freely relative to the rest of the organism. Animal joints have a limited range of motion (the angle they can move), but wheels need to be able to turn through any angle without needing to be turned back. Because of this, wheels can't be attached to the axle or shaft that they turn on. No true many-celled organism is known to grow solid tissues or organs that are not attached to the rest of the organism.