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.
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).