[aurelian15]

Without being an expert on the topic, I think it is safe to say that some symmetry in the nervous system is present throughout the entire animal kingdom -- just as bodies tend to be symmetric. For insects in particular, the brain is definitively split into two clearly distinguishable halves.

Octopuses in particular are a completely different story, they don't seem to possess a "central processing unit", but rather distribute necessary computations throughout their entire nervous system (it is kind of fascinating). Yet, since they have two eyes, you'll still find two symmetrical nuclei for processing visual information somewhere in the vicinity of the sensor system.