There are a few types of existing bilaterians that lack a recognizable brain, including echinoderms, tunicates, and acoelomorphs (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.
মস্তিষ্ক ছাড়া কিছু বাইলেটারিয়ান কি?
echinoderms, tunicates, and acoelomorphs