Evolution of nervous systems
Jellyfish, comb jellies, and related animals have diffuse nerve nets rather than a central nervous system. In most jellyfish the nerve net is spread more or less evenly across the body; in comb jellies it is concentrated near the mouth. The nerve nets consist of sensory neurons that pick up chemical, tactile, and visual signals, motor neurons that can activate contractions of the body wall, and intermediate neurons that detect patterns of activity in the sensory neurons and send signals to groups of motor neurons as a result. In some cases groups of intermediate neurons are clustered into discrete ganglia.
Pain in fish
In 2002, Rose published reviews arguing that fish cannot feel pain because they lack a neocortex in the brain. This argument would also rule out pain perception in most mammals, and all birds and reptiles. However, in 2003, a research team led by Lynne Sneddon concluded that the brains of rainbow trout fire neurons in the same way human brains do when experiencing pain. Rose criticized the study, claiming it was flawed, mainly because it did not provide proof that fish possess "conscious awareness, particularly a kind of awareness that is meaningfully like ours".