MEDUSA. 



85 



Fig. 52 represents the somewhat more complex 

 auditory organ of Phialidium. 



d^ 



Fig. 52.— Auditory organ of Pliialidium (after Hertwig). d', Epitlielium of tlie upper 

 surface of tlie velum ; d-, epithelium of the under surface of the velum ; hh, 

 auditory hairs ; /i, auditory cells ; 7?p, nervous cushion ; nr', nerve-ring ; r, 

 circular canal at the edge of the velum. 



The second type is more advanced, the vesicle being 

 closed, and the otolithes fewer in number, the Eucopidse, 

 indeed, having only one.* 



In the third type, that of the Trachymedusse, the 



Fig. 53.— Auditory organ of Rhopalonema, still showing a small orifice (after Hertwig). 

 hkf Modified tentacle ; o, auditory organ. 



auditory organs are modified tentacles. They form a 

 club-shaped body, with a central endodermal axis, and 



* Hertwig considers that the supposed hairs shown by Hensen in 

 his figure of the ear of Eucope are really the edges of auditory canals. 



