Amphioxi

Amphioxi populated the oceans about 550 million years ago, and they lived simple lives. Its meager nervous system included a teeny clump of cells that was not quite a brain. When you look at a modern amphioxus, you behold a creature very similar to your own ancient, tiny ancestor who roamed the same seas. The amphioxus didn't need a brain. Its cells for sensing were connected to its cells for moving, so it reacted to its meager senses without much processing. (Indebted to the thinking of Lisa Feldman Barrett)