In phylogeny they have been turned to the detection of gravitational forces, accelerations, and the precise analysis of sounds from distant sources.

Dijkgraaf (1263) has discussed the functional significance of this system, while Denison (1966) discussed the lateral line system and its relation to other vertebrate sense organs.

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Lateral line system is found in cyclostomes, fishes, aquatic amphibians, and aquatic larval stages of terrestrial amphibians, it is related to an aquatie mode of life.

The receptor organs are neuro­masts made of neurosensory cells and supporting cells, each neuro­sensory cell has a thin process or hair at its free end and a nerve fibre at the other end.

In cyclostomes, a few fishes, larval amphi­bians, and aquatic amphibians the neuromasts lie on the surface of the skin, especially on the head.

In most cartilaginous and bony fishes the neuromasts sink within the skin into depressions, grooves, or canals.

The canals contain mucus and open on the surface by many minute pores. The lateral line system is innervated by VII, IX and X cranial nerves.

Besides neuromasts the lateral line system has other receptors, they are pit organs and ampullae of Lorenzini in elasmobranchs, and pit organs in other fishes.

Lateral line receptors detect movements of water arid currents, they furnish information regarding the position of the body in relation to the environment and it is probable-that they detect vibrations of water.

In elasmobranchs the lateral line system has two long lateral line canals in the dermis, each running along the side of the entire body.

In the head region the two lateral canals are joined by a transverse occipital canal above th6 head, then each lateral line canal runs forward as a postorbital canal which divides into two branches, a supraorbital canal above the orbit and an infraorbital canal below the orbit, both run upto the snout.

Arising from the infraorbital canal is a jugal canal below the eye which runs back up to the first gill cleft; it gives off a mandibular canal to the lower jaw.

The canals are lined with epithelium having many mucous gland cells which secrete mucus, the canals open at intervals on the surface by vertical tubes (which pierce the scales in bony fishes).

The canals are filled with a fluid and mucus. In the canals are neuromasts, each made of a group of sensory receptor cells and supporting cells, each receptor cell has a stiff sense hair at one end and a nerve fibre at the other end, the hairs of receptor cells are tipped with a heavy gela­tinous substance.

The lateral line neuromasts are current receptors (rheoreceptors) detecting any vibrations of water. They are also found in eyclostomes, fishes and aquatic amphibians.

Pit organs are found on the dorsal and lateral surface of the head, they are ectodermal pits, each having a neuromast innervated by the VII cranial nerve.

Pit organs are scattered individual neuro­masts found in all fishes, they are rheorceptors,

Ampullae of Lorenzini are found only in elasmobranchs, they are highly modified receptors of the lateral line system, though some do not regard them as such.

The ampullae of Lorenzini are found in clusters on the dorsal and ventral side of the head embeded below the skin but opening externally on the surface of the skin.

Each ampulla has a pore opening on the surface, the pore leads into a duct or canal of Lorenzini filled with mucus, the canal ends below in an ampullary sac having 8 or 9 vertical chambers arranged radially around a central core or centrum.

The ampulla has mucous gland cells and sensory cells. The ampullae of Lorenzini are innervated by fibres of the facial (VII) nerve, they detect any changes of temperature (ther­moreceptors), but the deep position of sensory hair cells show that this is not their prime function, they also detect weak tactile stimulations and hydrostatic pressure at various depth. However, their real function remains uncertain.

Vesicles of Savi are like ampullae of Lorenzini but are found on the ventral surface only in the electric ray Torpedo as closed sac. A—Ampulla in the epidermis, their function is unknown of Lorenzini.

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