Definition: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: Segmented worms have a well-developed body cavity filled with fluid. This fluid-filled cavity serves as a hydroskeleton , a supportive structure that helps move the worm's muscles. Only the most primitive worms (the flatworms) lack a body cavity. Answer: hydroskeleton
Output:
What is the name of the supportive structed that helps move a worm's muscles?