Cephalaspis
Cephalaspis was a medium-sized fish that lived in freshwater streams and estuaries of the early Devonian. Some of the fossils are well-preserved, and so we know quite a bit about its anatomy. Its armour was on the dorsal side, and its mouth on the ventral side. Therefore, it is likely that it was a bottom-feeder, nosing about in the mud of a river. It had a typical vertebrate type of brain, with front, middle and rear regions, and nerves to and from sense organs and  muscles in the head region.