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.
Angling
In 2003, Scottish scientists at the University of Edinburgh performing research on rainbow trout concluded that fish exhibit behaviors often associated with pain, and the brains of fish fire neurons in the same way human brains do when experiencing pain. James D. Rose of the University of Wyoming critiqued the study, claiming it was flawed, mainly since it did not provide proof that fish possess "conscious awareness, particularly a kind of awareness that is meaningfully like ours". Rose argues that since the fish brain is rather different from ours, fish are not conscious, whence reactions similar to human reactions to pain instead have other causes. Rose had published his own opinion a year earlier arguing that fish cannot feel pain as they lack the appropriate neocortex in the brain. However, animal behaviorist Temple Grandin argues that fish could still have consciousness without a neocortex because "different species can use different brain structures and systems to handle the same functions." The position that Rose takes also fails to address unresolved empirical and philosophical considerations concerning pain, as raised by principles of epistemology, solipsism, existentialism, and comparative ethology. Until such problems are far more fundamentally resolved, there are strong arguments for refraining from causing the appearance of pain or behaviour consistent with pain, insofar as such things might be reasonably avoidable.
However, in 2012, a group of researchers led by Rose reviewed the literature, and concluded again that fish are not conscious and therefore do not feel pain.