Depends on your definition of thinking. If by think you mean react to their environment and possibly have some form of chemically based memory, that seems possible. Hell they have to react to the environment to maintain homeostasis. Multicellular organisms are ultimately a kind of colony of unicellular organisms if you get right down to it.

If you mean actual cognition, definitely not. There's an indisputable difference between reacting to stimulus and considering the cause of the stimulus and the second one needs a neutral network or something like it, it's simply too complex... Probably.

I also don't think it's unreasonable that an organ could affect thought patterns aka personalities, but there's a big, big difference between your brain's specific chemical environment they are affecting and the memories it contains.