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 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.
- Kandel ER, Schwartz JH, Jessel TM, eds. (2000). Ch. 17: The anatomical organization of the central nervous system. Principles of Neural Science. McGraw-Hill Professional. ISBN 978-0-8385-7701-1