Biology question: why do annelids have a more complex nervous system than do most flatworms (other than the planarian)?

Flatworms (apart from the turbellarians, (planarians are in the class turbellaria)) are parasitic and don't need complex nervous systems. A parasitic animal devotes much of itself to reproduction and doesn't worry about other tasks. Cestodes don't even have a gut, they absorb food via diffusion through their skin. Annelids are free-living animals, and they need to move around more. They don't spend their adult life hooked to the inside of an animal, they move around, and need a more complex nervous system to give them a better view of their world.

b brown mice / white sand dunes