Sponge
Like cnidarians (jellyfish, etc.) and ctenophores (comb jellies), and unlike all other known metazoans, sponges' bodies consist of a non-living jelly-like mass (mesoglea) sandwiched between two main layers of cells. Cnidarians and ctenophores have simple nervous systems, and their cell layers are bound by internal connections and by being mounted on a basement membrane (thin fibrous mat, also known as "basal lamina"). Sponges have no nervous systems, their middle jelly-like layers have large and varied populations of cells, and some types of cells in their outer layers may move into the middle layer and change their functions.
Pain in fish
Somatosensory evoked potentials (SEPs) are weak electric responses in the CNS following stimulation of peripheral sensory nerves. These further indicate there is a pathway from the peripheral nociceptors to higher brain regions. In goldfish, rainbow trout, Atlantic salmon ("Salmo salar") and Atlantic cod ("Gadus morhua"), it has been demonstrated that putatively non-noxious and noxious stimulation elicit SEPs in different brain regions, including the telencephalon which may mediate the co-ordination of pain information. Moreover, multiple functional magnetic resonance imaging (fMRI) studies with several species of fishes have shown that when suffering from putative pain, there is profound activity in the forebrain which is highly reminiscent of that observed in humans and would be taken as evidence of the experience of pain in mammals.