Structural and chemical features of the invertebrate otoliths

In the course of evolution of the invertebrate gravity receptor endogenous intracellular mode of the otoliths formation (Coelenterata, Ctenophora) is changed for endogenous extracellular mode (Mollusca, Arthropoda). The intracellular otolith is represented by cristalline concretions whereas extracellular one may consists of many separate crystallic otoconia or one large spherolith. As a rule in all cases qualitative mineral composition is similar. Calcium is the main component of the otolith in invertebrates (as vertebrates) though magnesium, sodium and potassium also take place in the otolith formation.