Chondrichthyes
Chondrichthyes nervous system is composed of a small brain, 8-10 pairs of cranial nerves, and a spinal chord with spinal nerves. They have several sensory organs which provide information to be processed. Ampullae of Lorenzini are a network of small jelly filled pores called electroreceptors which help the fish sense electric fields in water. This aids in finding prey, navigation, and sensing temperature. The Lateral line system has modified epithelial cells located externally which sense motion, vibration, and pressure in the water around them. Most subspecies have large well-developed eyes. Also, they have very powerful nostrils and olfactory organs. Their inner ears consist of 3 large semicircular canals which aid in balance and orientation. Their sound detecting apparatus has limited range and is typically more powerful at lower frequencies. Some subspecies have electric organs which can be used for defense and predation. They have relatively simple brains with the forebrain not greatly enlarged. The structure and formation of myelin in their nervous systems are nearly identical to that of tetrapods, which has led evolutionary biologists to believe that Chondrichthyes were a cornerstone group in the evolutionary timeline of myelin development.
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.