Nervous system
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, which pick up chemical, tactile, and visual signals; motor neurons, which can activate contractions of the body wall; and intermediate neurons, which detect patterns of activity in the sensory neurons and, in response, send signals to groups of motor neurons. In some cases groups of intermediate neurons are clustered into discrete ganglia.
Pain in fish
Other researchers also believe that animal consciousness does not require a neocortex, but can arise from homologous subcortical brain networks. It has been suggested that brainstem circuits can generate pain. This includes research with anencephalic children who, despite missing large portions of their cortex, express emotions. There is also evidence from activation studies showing brainstem mediated feelings in normal humans and foetal withdrawal responses to noxious stimulation but prior to development of the cortex.