424 



AUDITORY ORGANS OF THE CCELENTERATA. 



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. ^quorea, 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^) 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. Tiie 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 Trachymedusse, ap- 

 pears in its simplest condition as a modified tentacle. It is formed 



of a basal portion, covered 

 by auditory cells with 

 long stiff auditory hairs, 

 supporting at its apex a 

 club-shaped body.attacli- 

 ed to it by a delicate 

 stalk. An endodermal 

 axis is continued through 

 the whole structure, and 

 in one or more of the 

 endoderm cells of the 

 club-shaped body otoliths 

 are always present. Tlie 

 tails of the auditory cells 

 are directly continued in- 

 to the upper nerve-ring. 

 In more complicated 

 forms of this organ the tentacle becomes enclosed in a kind of cup, 

 by a wall-like upgrowth of the surrounding parts (fig. 298); and in 

 some forms, e.g. Geryonia, by the closure of the cup, the whole structure 

 takes the form of a completely closed vesicle, in the cavity of which 

 the original tentacle forms an otolith-bearing projection. 



The auditory organs found in the Acraspedote Medusije approach 

 in many respects to the type of organ found in the Trachymedusse. 

 They consist of tentacular organs placed in grooves on the under 

 surface of the disc. They have a swollen extremity, and are provided 

 with an endodermal axis for half the length of which there is a 

 diverticulum of the gastro-vascular canal system. The terminal por- 

 tion of the endoderm is solid, and contains calcareous concretions. 

 The ectodermal cells at the base of these organs have the form of 

 auditory cells. 



Mollusca. Auditory vesicles are found in almost all MoUusca on 



Fig. 298. Atiditoby organ ov Rhopalonema. 

 (From Lankester; after O.and R. Hertwig.) 



The organ consists of a modified tentacle {hk) 

 with auditory cells and concretions, partially en- 

 closed in a cup. 



