Atolla jellyfish
The body of "Atolla wyvillei" is a bell shape and it has several moderately long tentacles along the bell. The bell is around 20–174 mm in diameter. It also has a single long tentacle called a hypertrophied tentacle. This hypertrophied tentacle has several purposes, including aid in predation and aid in reproduction. These jellyfish do not have a digestive system, a respiratory system, a circulatory system or a central nervous system. Not having a central nervous system means that it lacks a brain.
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.