There are a few types of existing bilaterians that lack a recognizable brain, including <answer>echinoderms, tunicates, and acoelomorphs</answer> (a group of primitive flatworms). It has not been definitively established whether the existence of these brainless species indicates that the earliest bilaterians lacked a brain, or whether their ancestors evolved in a way that led to the disappearance of a previously existing brain structure.
Some bilaterians without a brain are what?
echinoderms, tunicates, and acoelomorphs