Lateral Line Organs

specialized sensory skin organs arranged in regular rows in the head and trunk of Cyclostomi, fish, amphibians living constantly in water, and the larvae of all amphibians.

With the lateral line organs the animals orient themselves to the velocity and direction of the current and to the movements of their own bodies. They also perceive currents reflected from solid objects; this enables them to go around objects in turbid water or in the dark and find food. Lateral line organs develop from ectodermal thickenings—placodes. They consist of cylindrical supporting cells surrounding pear-shaped sensory cells that have peculiar protuberances, or setae, at the upper free end are intertwined below with the terminal branches of the sensory nerve. The lateral line organs in the head are innervated by branches of the facial, glossopharyngeal, and trigeminal nerves. Lateral line organs in the trunk are innervated by the lateral branch of the vagus nerve. Lateral line organs of vertebrates originated from poorly differentiated skin organs—mechanoreceptors.

In Cyclostomi and amphibians lateral line organs are arranged on the surface of the skin in open grooves or separate shallow pits. In some fossil Agnatha, the lateral line organs lay in canals inside the shell plates. In primitive sharks and holocephalous fish, the skin forms deep grooves near the lateral line organs. In most fish the lateral line organs lie in grooves that are under the skin and communicate with the outside through openings. A lateral canal with short tubules that pierce the scales and open on their surface passes along the trunk. The openings of the tubules can be seen by the naked eye and form a lateral line. The canals of the lateral line in the head proceed to the opercular bones of the skull—the so-called canal bones—with which they are closely associated during development. The canals of the lateral line are located above and below the eye, along the anterior margin of the gill cover, and along the lower mandible. In the occipital region they are connected by a transverse bridge to the lateral line canal and to each other. In fossil amphibians, lateral line organs lay in the bone canals on the surface of the cranial bones, and only in Ichthyostegalia were they embedded in the bony canals, where they are located in fish.

Besides the ordinary lateral line organs, the skin of Euselachii contains Lorenzini’s ampullae, or long blind canals that end in sacculi. Each ampulla is divided by radial septa into sectors in which the lateral line organs rest. Near the electric organs of the electric ray are vesicles isolated from the outside—Savy sacs that contain lateral line organs.