P oriferans - ciliated sensory cells , no nervous system
E chinoderms - advanced version of C nidarians
P lanaria - cerebral ganglia ( concentrated nerve cells )
A nnelids - ventral nerve cord, starts from cerebral ganglion
A rthropods - ventral nerve cord
M ollusks - bivalves (3), gastropods (6), cephalopods (most complex)
V ertebrated - CNS and PNS
