Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Flatworms reflect several major evolutionary advances in invertebrates. They have three embryonic cell layers, including mesoderm. The mesoderm layer allows them to develop organ systems. For example, they have muscular and excretory systems. The muscular system allows them to move from place to place over solid surfaces. The excretory system lets them maintain a proper balance of water and salts. Flatworms also show cephalization and bilateral symmetry. Answer: excretory
What system lets flatworms maintain a proper balance of water and salts?