Answer
The main derived characteristics of chordates are seen in vertebrates, not in the adult but in embryonic stages. These are lost or modified when the adult body is finished developing.
Work Step by Step
The key here is to scan the text and see that the features of chordates are found in all chordates, but it’s a question of timing when you need to find them in vertebrates. Remember that embryology is key in understanding phylogenetic relationships.
