 Dear students, in this topic we shall discuss the organs of equilibrium in animals. The organs of equilibrium detect animals position with respect to gravity and acceleration. The simplest organ involved in equilibrium is the Statocyst which is found in many invertebrates. A Statocyst is a simple organ which consists of a fluid filled cavity. This cavity is lined with ciliated mechanoreceptor cells. At the cavity there is a solid particle which is called Statolyt. This form of organ is found in most of the invertebrates except a major group which is insects, they do not have Statocysts. Dear students, the Statocysts work when the position of an animal changes the Statocyst has having the Statolyt, the Statolyt moves with gravity. When it moves it strikes with the mechanoreceptor cells of the Statocyst cavity. As a result these mechanoreceptors are activated. When the mechanoreceptors are activated they generate signals which travel to the central nervous system. Central nervous system appendages go, limbs go, messages send, as a result they change their position with reflex movements or as a result equilibrium or balance of the body is established. Dear students, the organ of equilibrium in vertebrates is called the vestibular apparatus. This vestibular apparatus is located inside the inner ear. It consists of three major components, saccule, utricle and semicircular canals. The inner surfaces of saccule and utricles have a sensory area which is called macula. This macula is covered by a gelatinous layer. This gelatinous layer has many small crystals of calcium carbonate called atolites or Statoconia. This pattern is similar to the Statocyst. It is the simplest organ and it is an advanced organ. Now the macula has thousands of hair cells. These hair cells have celia which project into the gelatinous layer. Atolites or Statoconia move in response to gravity. They cause the celia to bend as a result signals are generated which are transmitted through the vestibular nerve to the central nervous system. The saccule and utricles maintain the static equilibrium of the head. The macula of the utricle lies in the horizontal plane on the inferior surface. This is located on the horizontal plane on the lower surface of the utricle. This determines the orientation of the head in the upright position. The macula of the utricles is located in the vertical position due to which it determines the head's orientation in the line pose. When we are lying down, the saccule determines the position of the head. Now we shall discuss the role of semicircular canals. There are three semicircular canals which are laying orthogonally that is perpendicular to each other. Three semicircular canals are at right angles from each other. These canals are filled with a fluid which is called endolymph. Every canal has an enlargement at one end. This enlargement is called ampula. Each ampula has a small crest which is covered with a loose gelatinous tissue mass. Every crest has a loose gelatinous mass. This is also called tissue mass and this tissue mass is called copula. Hair cells which are located on the ampullary crest project their celia into the copula. These hair cells detect the acceleration of the head in the semicircular canals. The imbalances with the head during the head movement detect semicircular canals. Because they are orthogonally arranged, they can detect any movement of the head in any of the planes. Then when head rotates, it results in the flow of endolymph inside the semicircular canals. As a result, this endolymph strikes with copula and the hair cells in the copula are displaced. Displacement of celia results in the excitation of hair cells. When excitation occurs, it causes membrane potential to change and generate a signal.