Anatomy of finfish and shellfish

Unit 7 - Nervous system in fishes

7.2. Nerves
Apart from the brain and the spinal cord the fish body is supplied with a vast network of nerves, the electric wires of the body along which messages travel. Nerves are built of of numerous neurons and neurons are a one-way system, messages either travel to or from the brain or the spinal cord along a particular neuronal path, but never both ways. Those nerves that arise from the spinal cord are called spinal nerves and those which arise from the brain are called cranial nerves.
Normally there is one pair of spinal nerves (left and right) for each vertebrae, thus long thin fish with many vertebrae such as eels will have many more pairs of spinal nerves than a much shorter fish such as a gobi. In fish there are 10 pairs of cranial nerves all with well defined roles.