In Shark 3 pair of internal ears are present. They are organs of equilibrium and hearing. The middle and external ears are absent. The internal ear is a complex structure. It is called the membranous labyrinth. It is present in the auditory capsule of the cranium. It is covered by cartilaginous labyrinth. A space is present between the membranous and cartilaginous labyrinths. It is called peri lymphatic space. It contains peri lymph. It protects and transmits vibrations to the membranous labyrinth. Connective tissue extends from the Cartilaginous to the membranous labyrinth.
The membranous labyrinth has laterally compressed vestibule which is divided into two chambers,
a) The upper narrow utricles,
b) Lower wide sacculus.
The two chambers are connected with each other by a wide passage, the sacculoutricular duct. The sacculus gives off a conical projection from its lower side, called lagena. A narrow canalarisesfrom the dorsal side of the sacculus called ductus endolymphaticus. It pierces through the roof of the cranium and open out by a minute pore. Before it opens out the duct dilates into saccus endolymphaticus. Three semicircular ducts are present. They are called the external, anterior and posterior semicircular canals. . The external canal is horizontal and the other two are vertical in position. All the semicircular anals open into the utriculus at both of their ends. One end of each canal is dilated into oval ampulla. The ampullae of the anterior and the external canals lie close together towards the anterior end of the ear.
The cavity in the membranous labyrinth is filled with a fluid Endo lymph. Six sensory patches are present in the wall of the internal ear.The sensory patches' of the ampullae are called the "cristae". The sensory patches of the utriculus, sacculus and lagena are called maculae acousticae They contain neurosensory and supporting cells. Each neurosensory cell bears a sensory hair at the free end, and passes into a nerve fiber at the other end. The nerve fibres join the auditory nerve. The sensory hair are much longer cristae than in the maculae. They are enclosed in a gelatinous substance. The gelatinous substance is called the cupule. In Maculae the otolithic membrane has in its outer part minute crystalline bodies called otoliths.
They are formed of calcium carbonate.
- The cristae detect turning movements of the head.
- The maculae in the utriculus and sacculus detect changes in the position of the body at rest and in the forward movement. With a change in the position of the body, the otoliths change their position due to gravity and bring pressure on the underlying sensory hairs and this stimulates the neurosensory cells. The body position is corrected. Thus equilibrium is brought forward.
- The maculae & lagena will detect under water sound vibrations to some extent.