You are presented with the quiz "Starfish, sea urchins and sea cucumbers lack a what?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "echinoderms lack a centralized nervous system" and "echinoderms include starfish, sea urchins, and sea cucumbers". 

So, what's the best answer to the question?


 

    centralized nervous system       