All multicellular organisms have a nervous system, which may be defined as assemblages of cells specialized by their shape and function to act as the major coordinating organ of the body. Nervous tissue underlies the ability to sense the environment, to move and react to stimuli, and to generate and control all behavior of the organism. Compared to vertebrate nervous systems, invertebrate systems are somewhat simpler and can be more easily analyzed. Invertebrate nerve cells tend to be much larger and fewer in number than those of vertebrates. They are also easily accessible and less complexly organized; and they are hardy and amenable to revealing experimental manipulations, such as changes in the composition and temperature of the fluids surrounding them. However, the rules governing the structure, chemistry, organization, and function of nervous tissue have been strongly conserved phylogenetically. Therefore, although humans and the higher vertebrates have unique behavioral and intellectual capabilities, the underlying physical-chemical principles of nerve cell activity and the strategies for organizing higher nervous systems are already present in the lower forms. Thus neuroscientists have taken advantage of the simpler nervous systems of invertebrates to acquire further understanding of those processes by which all brains function.