MEDUSJE. 



85 



Fig. 52 represents the somewhat more complex 

 auditory organ of Phialicliiim. 



rf^_ 



Fig. 52.— Auditory organ of Plilalidium (after Hertwig). d\ Epitlielium of tlie upper 

 surface of tlie velum ; d\ epitlielium of the under surface of the velum ; lih, 

 auditory hairs ; h, auditory cells ; vp, 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 Trachy medusae, the 



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

 hk, 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 Hansen in 

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



