Answer
Flatworms have protonephridia, earthworms have metanephridia, and insects have Malpighian tubules.
Work Step by Step
Flatworms and earthworms both remove waste products from interstitial fluid through ciliated cells/channels to the proto- or metanephridia, respectively. Insects remove waste products from interstitial fluid through Malpighian tubules into the excretory end of their digestive systems.