You are presented with the quiz "What lack respiratory and excretory systems?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "echinoderms lack respiratory and excretory systems" and "echinoderms in the diorama include the crinoids and blastoids". 

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


 

     crinoids and blastoids      