Detailed Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Problem:Passage: Lancelets are invertebrate chordates that retain all four defining chordate traits as adults. There are only about 25 species of living lancelets. Lancelets resemble tunicates in several ways. For example:. Answer: lancelets
Solution:
What are invertebrate chordates that retain all four defining chordate traits as adults called?