Introductory SurveyAuthor(s): O. LowensteinSource: Proceedings of the Royal Society of London. Series B, Biological Sciences, Vol. 152, No.946, A Discussion on the 'Ear' Under Water (Apr. 26, 1960), pp. 1-2Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/75358 .

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A DISCUSSION ON THE 'EAR' UNDER WATER

UNDER THE LEADERSHIP OF O. LOWENSTEIN, F.R.S.

(Discussion held 12 March 1959-Received 15 July 1959)

[Plates 1 to 11]

CONTENTS PAGE

0. LOWENSTEIN, F.R.S. Introductory survey 1

J. Z. YOUNG, F.R.S. The statocysts of Octopus vulgaris 3 M. J. COHEN The response patterns of single receptors in

the crustacean statocyst 30 P. VIGOUREUX Underwater sound 49

S. DIJKGRAAF Hearing in bony fishes 51

F. W. REYSENBACH DE HAAN Some aspects of mammalian hearing under water 54

F. C. FRASER and P. E. PunVES Anatomy and function of the cetacean ear 62

Introductory survey

BIY O. LOWENSTEIN, F.R.S.

Department of Zoology and Comparative Physiology, University of Birmingham

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 inhomo- geneities 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.

I [ 1 ] Vol. I52, B. (26 April 1960)

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O. Lowenstein (Discussion meeting)

As we are going to hear accounts by Professor J. Z. Young and Dr Melvin Cohen on the form and function of the statocyst in the Mollusca and Crustacea, I shall confine myself to a few historical remarks concerning these sense organs. They were

originally known as otocysts and they were considered to be organs of hearing. However, after the discovery of the equilibrium function of the otolith organs of the vertebrate ear, opinion veered towards the assumption of a static function for all 'lith'-bearing sense organs, and this led to a renaming of otocysts into statocysts. At the same time it became generally accepted that animals lacking an organ corresponding in design to the cochlea of the higher vertebrates are devoid of the

power of sound reception altogether. No direct line of evolutionary homology can with certainty be established be-

tween the invertebrate statocysts and the structures of the inner ear in vertebrates. It looks as if the story of the vertebrate ear starts with the lateral line. It may be assumed that cupula-bearing hair cells serving as flow indicators were first arranged peripherally in the integument of fishes, where they are still found in some forms.

They then became lodged in systems of subepidermal tubes to form the lateral line

organs. Pumphrey (1950) assumed that not only the cupula-bearing end organ of the semicircular canals of the labyrinth, but also the otolith organs with sensory hair processes ensheathed in lime-encrusted otolith membranes, were direct de- rivatives of the lateralis organs. Such weighted covering structures constitute a

physical system suitable for the detection of changes in acceleration, and it is therefore not surprising to find receptor organs for gravitational stimuli and linear acceleration closely side by side with sound receptors. Sensitivity to displacement in the field of gravity and vibration sensitivity have been electrophysiologically demonstrated to exist in different regions of one and the same otolith organ (Lowenstein & Roberts I95I).

In the higher vertebrates the ear, having been first evolved in aquatic animals, goes through interesting stages of adaptation connected with the transition to life on land. The special matching devices of the middle ear and the outer ear with its function of sound localization were evolved. We are, however, not interested in this part of the evolutionary history of the ear. What we are concerned with are the adaptive changes that became necessary when the ear, fully adapted for the reception of airborne sound, became immersed again in a fluid medium, when some mammals returned to an aquatic mode of life. Reports on recent work on underwater hearing in the Cetacea carried out both in Holland by Dr Reysenbach de Haan, and here at the British Museum by Dr Fraser and Mr Purves will show the functional adaptations which have taken place in and around the mammalian ear to re-equip it for a task for which it was originally evolved. The special problems arising in connexion with sound reception under water stem from the physical peculiarity of sound propagation in a fluid medium. These will be dealt with in a

paper by Dr Vigoureux.

REFERENCES (Lowenstein)

Lowenstein, O. E. & Roberts, T. D. M. 1951 J. Physiol. 114, 471-489. Pumphrey, R. J. 1950 Symp. Soc. Exper. Biol. 4, 3-18.

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