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.
Jellyfish as food
In China, some species of jellyfish in the Rhizostomae order caught in coastal areas have been utilized as a source of food and ingredient in Chinese cuisine for over 1,700 years. Cannonball jellyfish ("Stomolophus meleagris") and jelly blubber ("Catostylus mosaicus") are edible species of jellyfish. When live, the cannonball jellyfish contains toxins that can cause cardiac problems in animals (including humans). "Rhopilema esculentum" and "Rhopilema hispidium" are edible jellyfish, and are the most common ones consumed in China, Japan and Korea. Additional edible jellyfish species include "Aurelia aurita", "C. orsini", "Dactylometra pacifica", "Hispidium spp.", "Lobonema smithi", "Lobonemoides gracilis" and Nomura's jellyfish ("Stomolophus nomuria"). Desalted ready-to-use jellyfish are low in calories and contain hardly any fat, about 5% protein and 95% water. They do not have much flavor, and may be used to add additional texture and mouthfeel to various dishes. In some areas of Asia, jellyfish is "associated with easing bone and muscle pain."