460 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



(which belong, of course, to the series of cells that 

 lie on the upper surface of the velum) have thick 

 membranes, and become filled with fluid. Mitrotrocha 

 is provided with no less than eighty such open pits. 



The next stage in advance is seen in Phialidium 

 (Fig. 193), where the pit becomes closed. In other 

 Medusae the auditory vesicle appears to be a modified 



Fig. 193. An Auditory Vesicle of Phialidium. 



d', Epithelium of the upper surface of the velum ; eft, of the under surface ; 

 nr', upper nerve ring; ft, auditory cells; hh, auditory hairs ; np, nervous 

 cushion formed by a prolongation of the lower nerve ring ; r, circular canal 

 at the edge of the velum. The; spot in the cavity represents an otolith. 

 (After 0. and R. Hertwig.) 



tentacle. In some cases the sense organs appear to be 

 both eye and ear. 



Throughout the whole of the Metazoa we find 

 that the auditory organs have more or less the form 

 of vesicles, which in lowlier forms, and in all primi- 

 tively, are open to the exterior ; within the vesicles 

 one or more hard bodies are developed, which, on being 

 agitated by the vibrations of the waves of sound, act 

 on the sensory hairs of the sense cells, which are 

 developed within the auditory cavity, and which are 

 connected with the central nervous system by the 

 fibres of the auditory nerve. 



