Which feature does not characterize a protonephridium?

Question

Quizplus · Accepted Answer

Protonephridia do not rely on blood pressure to drive filtration. They use cilia to create a current that draws fluid into the tubules, where the flame cells actively transport ions and other substances to modify the composition of the filtrate. The filtrate then flows toward the excretory pore and is eliminated from the body. The urine produced by protonephridia is also typically more concentrated than extracellular fluid, which helps conserve water in terrestrial animals.

Which Feature Does Not Characterize a Protonephridium