Jellyfish
Most jellyfish do not have specialized systems for osmoregulation, respiration and circulation, and do not have a central nervous system. Nematocysts, which deliver the sting, are located mostly on the tentacles; true jellyfish also have them around the mouth and stomach. Jellyfish do not need a respiratory system because sufficient oxygen diffuses through the epidermis. They have limited control over their movement, but can navigate with the pulsations of the bell-like body; some species are active swimmers most of the time, while others largely drift.
The rhopalia contain rudimentary sense organs which are able to detect light, water-borne vibrations, odour and orientation. A loose network of nerves called a "nerve net" is located in the epidermis. Although traditionally thought not to have a central nervous system, nerve net concentration and ganglion-like structures could be considered to constitute one in most species. A jellyfish detects stimuli, and transmits impulses both throughout the nerve net and around a circular nerve ring, to other nerve cells. The rhopalial ganglia contain pacemaker neurones which control swimming rate and direction.
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.