514 AUDITORY ORGANS OF THE CCELENTERATA. 



of the velum with their apertures directed downwards. The 

 majority of the cells lining the outer, i.e. peripheral side of the 



FIG. 297. AUDITORY VESICLE OF PHIALIDIUM AFTER TREATMENT WITH 

 DILUTE OSMic ACID. (From Lankester; after O. and R. Hertwig.) 



d l . epithelium of the upper surface of the velum; d z . epithelium of the under 

 surface of the velum ; r. circular canal at the edge of the velum ; nr 1 . upper nerve- 

 ring ; h. auditory cells ; hh. auditory hairs ; np. nervous cushion formed of a 

 prolongation of the lower nerve-ring. Close to the nerve-ring is seen a cell, shewn as 

 black, containing an otolith. 



pit, contain an otolith, while a row of the cells on the inner, i.e. 

 central side, are modified as auditory cells. The auditory cells 

 are somewhat strap-shaped, their inner ends being continuous 

 with the fibres of the lower nerve-ring, and their free ends being 

 provided with bent auditory hairs, which lie in contact with the 

 convex surfaces of the cells containing the otoliths. 



By the conversion of such open pits into closed sacks a more 

 complicated type of auditory organ, which is present in many of 

 the Vesiculata, viz. ^Equorea, Octorchis, Phialidium, &c., is 

 produced. A closed vesicle of this type is shewn in fig. 297. 

 Such organs form projections on the upper surface of the velum. 

 They are covered by a layer of the epithelium (d 1 } of the upper 

 surface of the velum, but the lining of the vesicle (d*) is derived 

 from what was originally part of the epithelium of the lower 

 surface of the velum, homologous with that lining the open pits 

 in the type already described. The general arrangement of the 

 cells lining such vesicles is the same as that of the cells lining 

 the open pits. 



A second type of auditory organ, found in the Trachyme- 

 dusae, appears in its simplest condition as a modified tentacle. 



