There are a few types of existing bilaterians that lack a recognizable brain, including echinoderms, tunicates, and <answer>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.
A name for a group of primitive flatworms is what?
acoelomorphs