Ganglion

In anatomy, a ganglion (plural ganglia) is a mass of tissue in the nervous system. It is a group of nerve cells which do a job in the nervous system. Ganglia are basically clusters of nerve cell bodies. They may have thousands of nerve cell bodies working on some particular function. We perceive the brain's action as a whole. We cannot perceive ("see") the activity unless it goes wrong.
In the brain, the basal ganglia is a group of nuclei interconnected with the cerebral cortex, thalamus, and brainstem. They do various things: motor control, cognition, emotions, and learning.
In invertebrates, ganglia often do the work of a brain. In cases like the earthworm, there is a ganglion above the gut at the front. This is linked to another under the gut by nerve fibres running down each side of the gut. The rest of the central nervous system runs under the gut. This type of arrangement in found in a number of invertebrate phyla, and contrasts with the vertebrates, who have their spinal cord above (dorsal to) their gut.
In another usage, ganglion cells are found in the retina of the vertebrate eye.