Which characteristic is NOT true for nematodes?

Nematodes are distinguished by their unique body plan, which includes only longitudinal muscles and no circular muscles. This muscle arrangement contributes to their characteristic thrashing movement. The other options accurately describe nematodes: they indeed possess a complete digestive system, which allows for one-way digestion from mouth to anus; they include many parasitic species, such as hookworms and roundworms; and they play significant roles in ecosystems, particularly in decomposition and nutrient cycling. However, the complexity of muscle types in nematodes solely involves longitudinal muscles, affirming that the statement about having both circular and longitudinal muscles is not true for this group.