Why do annelids have a more complex nervous system than do most flatworms?

Answer 1

Many flatworms are merely parasites and often do not even have a gut for processing food. They simply live in the intestine of a host and absorb nutrients there. Many flat worms pass from one organism to the other through fecal matter with ease and do not need a complicated system for movement hence no need for a complex nervous system for moving.

Annelid worms are more often predators or consumers of organic material and require a digestive system. Further some annal id worms have to move around in their environment i.e. the earthworm that digs tunnels or marine worms that search for small animals to eat. All this requires the Annelid worm to have a more complex nervous system to digest and move around.