Kinocilium

A kinocilium is a special structure connected to the hair cells of the inner ear's cochlea. It acts to aid in depolarization and hyperpolarization of the plasma membrane due to bending of stereocilia.

Anatomy in humans
In the human ear, each hair cell has an especially tall cilium, called the kinocilium. Cochlear hair cells have kinocilia that degenerate during fetal development, indicating these processes play no essential role in transduction; instead kinocilia may be important in delineating the anatomical asymmetry of stereocilia or in making some mechanical connections with the hair cells in which they persist.

Anatomy in fish and frogs
The apical surface of a sensory fish hair cell usually has numerous stereocilia and a single, much longer kinocilium. Deflection of the stereocilia toward or away from the kinocilium causes an increase or decrease in the firing rate of the sensory neuron innervating the hair cell at its basal surface.

Hair cells in fish and some frogs are used to detect water movements around their bodies. These hair cells are embedded in a jelly-like protrusion called cupula. The hair cells therefore can not be seen and do not appear on the surface of skin of fish and frogs.