What is the simplest organism that demonstrates the development of a head brain?

Hmmm... That's one of those questions that may not have an absolute answer, but to be honest, I'm not sure.

 

It's possible that a more refined question (like what was the first organism to display synapses, or what was the first organism to have a protospine, etc.) could have a more refined answer.

 

Here's an interesting .pdf that might help you put some context around your terms:

 

http://www.ijdb.ehu.es/ijdb20034778/ft555.pdf

 

The origin and evolution of the nervous system

 

 

ABSTRACT: The nervous systems of animals as diverse as flies and mice share many conserved features, suggesting that such features were already present in their last common ancestor. As our knowledge of neural development increases, so does the list of conserved features, pointing to the existence of a highly sophisticated, single species as the origin of most extant nervous systems. Possible reasons for this unexpected monophyly are discussed, leading to the conclusion that the appearance of very different life forms, lifestyles and habitats requires the previous attainment of a neural circuitry that is sufficiently robust to cope with large changes without losing its overall coherence.