The lateral line organ is only found in fish and a few primitive amphibians. It is a narrow canal that runs down the side of a fish underneath the scales, from head to tail. A system of lateral line organs can be seen as a row of small pores along the fish. The pores are not restricted to the lateral line but are also distributed all over the fish, particularly on the head.

The individual pores, called a lateral line organ, or neuromas, consists of a cluster of hairs surrounded by a jellylike substance that flexes in response to pressure. The pores serve to detect pressure changes in the surrounding water. Pressure waves, like those caused sound or moment, excite the neuromas via the gel and inform the brain.

A swimming fish sets up its own pressure wave in the water that is detectable by the lateral line systems of other fish. It also sets up a bow wave in front of itself, the pressure of which is higher than that of the wave flow along its sides. The differences in the two pressures are registered by its own lateral line system. As the fish approaches an object, such as a rock or the glass wall of an aquarium, the pressure waves around its body are distorted, and these changes are quickly detected by the lateral line system, enabling the fish to swerve or to take other suitable action.