Nervous system
Nervous systems are found in most multicellular animals, but vary greatly in complexity. The only multicellular animals that have no nervous system at all are sponges, placozoans, and mesozoans, which have very simple body plans. The nervous systems of the radially symmetric organisms ctenophores (comb jellies) and cnidarians (which include anemones, hydras, corals and jellyfish) consist of a diffuse nerve net. All other animal species, with the exception of a few types of worm, have a nervous system containing a brain, a central cord (or two cords running in parallel), and nerves radiating from the brain and central cord. The size of the nervous system ranges from a few hundred cells in the simplest worms, to around 300 billion cells in African elephants.
Pain in fish
In papers published in 2017 and 2018 Michael Woodruff summarized a significant number of research articles that, in contradiction to the conclusions of Rose and Key, strongly support the hypothesis that the neuroanatomical organization of the fish pallium and its connections with subpallial structures, especially those with the preglomerular nucleus and the tectum, are complex enough to be analogous to the circuitry of the cortex and thalamus assumed to underlie sentience in mammals. He added neurophysiological and behavioral data to these anatomical observations that also support the hypothesis that the pallium is an important part of the hierarchical network proposed by Feinberg and Mallatt to underlie consciousness in fishes.