Sponge
Like cnidarians (jellyfish, etc.) and ctenophores (comb jellies), and unlike all other known metazoans, sponges' bodies consist of a non-living jelly-like mass (mesoglea) sandwiched between two main layers of cells. Cnidarians and ctenophores have simple nervous systems, and their cell layers are bound by internal connections and by being mounted on a basement membrane (thin fibrous mat, also known as "basal lamina"). Sponges have no nervous systems, their middle jelly-like layers have large and varied populations of cells, and some types of cells in their outer layers may move into the middle layer and change their functions.
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.