Introductory survey

Abstract

When we speak of the ear we associate it primarily with hearing. In our symposium today we shall, however, not be exclusively concerned with hearing in aquatic animals; and this has to do with the evolutionary history of the ear. Even in its most highly evolved form the ear is still a dual- or better-to-say a triple-purpose sense organ. In it are combined receptors for angular acceleration (the semicircular canals) for linear, including gravitational, acceleration (the otolith organs) and an organ for the accurate frequency analysis of sound (the organ of Corti in the cochlea). The otolith organs, are by dint of their particular design capable of signalling oscillatory changes in linear acceleration manifesting themselves as vibration or as sound in the widest meaning of the term. The functional association between reception of gravitational and vibrational stimuli, between balance and hearing is, phylogenetically speaking, a very old story. It begins probably with the emergence of the living cell. There exist inhomogeneities in density in the form of various cell inclusions, such as food vacuoles, etc., in an otherwise gravitationally or vibrationally ‘transparent’ cytoplasmic system. A density of two or three times that of water is sufficient to give pivotal importance to such cell inclusions making them capable of acting as internal points of reference for the perception of accelerational changes by the pressure-sensitive surrounding cytoplasm. This, on the multicellular level of organization, is the functional principle of the invertebrate statocyst.