AUDITORY ORGANS. 515 



It is formed of a basal portion, covered by auditory cells with 

 long stiff auditory hairs, supporting at its apex a club-shaped 

 body, attached 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. The tails of the auditory cells are directly continued 

 into 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 



FIG. 298. AUDITORY ORGAN OF RHOPAI.ONEMA. (From Lankester ; after O. 

 and R. Hertwig.) 



The organ consists of a modified tentacle (hk) with auditory cells and con- 

 cretions, partially enclosed in a cup. 



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 Medusae 

 approach in many respects to the type of organ found in 

 the Trachymedusae. 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 portion 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 Mol- 

 lusca on the ventral side of the body in close juxtaposition to 

 the pedal ganglia. Except possibly in some Cephalopods, these 



332 



