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.
Neuroanatomy
Aside from the human brain, there are many other animals whose brains and nervous systems have received extensive study as model systems, including mice, zebrafish, fruit fly, and a species of roundworm called "C. elegans". Each of these has its own advantages and disadvantages as a model system. For example, the "C. elegans" nervous system is extremely stereotyped from one individual worm to the next. This has allowed researchers using electron microscopy to map the paths and connections of all of the approximately 300 neurons in this species. The fruit fly is widely studied in part because its genetics is very well understood and easily manipulated. The mouse is used because, as a mammal, its brain is more similar in structure to our own (e.g., it has a six-layered cortex, yet its genes can be easily modified and its reproductive cycle is relatively fast).