Acoelomorphan Development:
There is one thing about the acoelomorphs that is quite distinctive, and that is how they develop. The early
cleavage patterns in acoelomorphs differ from those of deuterostomes or other protostomes. Because of this, some researchers believe that the ancestors of modern acoelomorphs split off from the main line of bilaterian evolution before the great split into protostomes and deuterostomes happened. If this is the case, it lends support to the notion that the acoelomorphs are the oldest surviving lineage of bilaterally-symmetrical animals.
Flagellophora apelti
, a common marine acoelomorph.