Can you describe a major innovation in body plan that first occured in roundworms?

A major innovation in the body plan of roundworms (nematodes) is the development of a pseudocoelom, a fluid-filled body cavity that lies between the endoderm and mesoderm. This structure provides structural support and allows for greater flexibility and movement, facilitating more efficient nutrient distribution and waste removal. The pseudocoelom also plays a crucial role in the circulatory and digestive systems, distinguishing roundworms from more primitive organisms with solid body plans.