Although neurons may differ in shape and size, they are essentially the same from the most primitive animal life to the most advanced. It is their number and organization that differ: we have a hundred billion nerve cells, while a jellyfish has a thousand. But their status as cells capable of rapid and repetitive firing is essentially the same.
It is fascinating to think of Darwin, Romanes, and other biologists of their time searching for “mind,” “mental processes,” “intelligence,” even “consciousness” in primitive animals like jellyfish, and even in protozoa. A few decades afterward, radical behaviorism would come to dominate the scene, denying reality to what was not objectively demonstrable, denying in particular any inner processes between stimulus and response, deeming these as irrelevant or at least beyond the reach of scientific study.