Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: Some animal-like protists use their "tails" to eat. These protists are called filter-feeders . They acquire nutrients by constantly whipping their tails, called flagellum , back and forth. The whipping of the flagellum creates a current that brings food into the protist. Answer: filter-feeders
Output:
Protists which use their tails to eat are called what?