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.
Models of neural computation
Mutually inhibitory processes are a unifying motif of all central pattern generators. This has been demonstrated in the stomatogastric (STG) nervous system of crayfish and lobsters. Two and three-cell oscillating networks based on the STG have been constructed which are amenable to mathematical analysis, and which depend in a simple way on synaptic strengths and overall activity, presumably the knobs on these things. The mathematics involved is the theory of dynamical systems.Flight control in the fly is believed to be mediated by inputs from the visual system and also the halteres, a pair of knob-like organs which measure angular velocity. Integrated computer models of "Drosophila", short on neuronal circuitry but based on the general guidelines given by control theory and data from the tethered flights of flies, have been constructed to investigate the details of flight control.