Structural diversity of the ordinary and specialized lateral line organs

Lateral line organ, a superficial sensory system in amphibia and fish which provides the animal with information about its surrounding environment, is divided classically into two main different types, ordinary and specialized, whose functions are mechanoreceptive and electroreceptive, respectively. Although it has great diversity, the basic sensory unit, which is usually called "neuromast," is composed of sensory cells embedded in accessory cells. The functions of the latter are to support the sensory cells and to secrete the material that covers the organs, forming a cupular structure or filling a canal which enables the organ to communicate with the exterior. Sensory cells of mechanoreceptive neuromasts have a tuft of processes included in the cupular material; these are a kinocilium and a group of stereocilia with a typical staircase arrangement. The displacement of the stereocilia towards or away from the kinocilium produces different stimuli. The electroreceptive organs are more diverse. They include ampullary and tuberous organs. The latter can be subdivided into different types: knollenorgans, mormyromasts, gymnarchomasts, etc. All of these present a great diversity among species, but their morphology is less reported than that of the mechanoreceptive organs. This paper summarizes the structural features of the main different types of lateral line organs, as well as their taxonomic distribution and different patterns of distribution along the surface of the animal.