Earthworms structural adaptations are its setae?

Yeah, that's right. Setae are tiny bristles which cover the wormy and they help wormy to grip the soil that it passes through. Thus, they can make a quick escape easily and also, they can find the right decaying matter (which they use as food) quickly. That's because if wormy grips the soil as it passes it won't slip around. So it can move in any direction happily.