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.
Pescetarianism
Furthermore, pescetarianism may be perceived as more ethical because fish, along with certain other animals such as insects, may not associate pain and fear as more complex animals like mammals do. Researchers have found that, unlike mammals, fish do not have the neuro-physiological capacity for a conscious awareness of pain. Fish do not possess a neocortex, which is the first indicator of doubt regarding whether they have pain-awareness. That is, certain nerve fibres in mammals (known as c-nociceptors) involved in the sensation of intense experiences of pain are not present in primitive cartilaginous fish. Some cartilagenous fish, such as sharks and rays do contain traces, yet there is an incomplete development of these fibres. To further test this, researchers administered powerful painkillers (such as morphine, which are highly effective in humans and mammals) to fish. They were found to be either totally ineffective or were only partially effective in doses so high that they would result in death from overdose. In this respect, although fish do show instinctive reactions to injuries and other interventions, the physiological prerequisites for the conscious experience of pain is not present. This, in combination with the pharmacological data, has supported the notion that fish do not feel pain in human, mammalian or biological terms. However, this theory is disputed: see the main article Pain in fish.