Hypothetical scenario of evolution of the lophophore nervous system in phoronids and bryozoans. Protophoronid ancestor had two nerve centres and two nerve rings: inner (cyan) and outer (purple). In bryozoans, these nerve centres fused each other forming cerebral ganglion, which exhibits zonality. The outer nerve ring underwent reduction and had lost. In phoronids, inner nerve centres and inner nerve ring were reduced in adults, but retained in Phoronis ovalis and maintained in part in juvenile phoronids.