[191] The most detailed account is that of W. Müller (No. [377]).
[192] Degeneration, London, 1880, p. 49.
[193] F. Leydig. “Ueber Nebenaugen d. Chauliodus Sloani.” Archiv f. Anat. und Phys., 1879. M. Ussow. “Ueb. d. Bau d. augenähnlichen Flicken einiger Knochenfische.” Bul. d. la Soc. d. Naturalistes de Moscou, Vol. LIV. 1879. Vide for general description and further literature, Günther, The Study of Fishes, Edinburgh, 1880.
CHAPTER XVII.
AUDITORY ORGAN, OLFACTORY ORGAN AND SENSE ORGANS OF THE LATERAL LINE.
Auditory Organs.
A great variety of organs, very widely distributed amongst aquatic forms, and also found, though less universally, in land forms, are usually classed together as auditory organs.
In the case of all aquatic forms, or of forms which have directly inherited their auditory organs from aquatic forms, these organs are built upon a common type; although in the majority of instances the auditory organs of the several groups have no genetic relations. All the organs have their origin in specialized portions of the epidermis. Some of the cells of a special region become provided at their free extremities with peculiar hairs, known as auditory hairs; while in other cells concretions, known as otoliths, are formed, which appear often to be sufficiently free to be acted upon by vibrations of the surrounding medium, and to be so placed as to be able in their turn to transmit their vibrations to the cells with auditory hairs[194]. The auditory regions of the epidermis are usually shut off from the surface in special sacks.
The actual function of these organs is no doubt correctly described, in the majority of instances, as being auditory; but it appears to me very possible that in some cases their function may be to enable the animals provided with them to detect the presence of other animals in their neighbourhood, through the undulatory movements in the water, caused by the swimming of the latter.