278 AUDITORY ORGANS. 



period another fold may appear forming the eyelids (fig. 122 C, 

 Pal). 



Auditory organs. A pair of auditory sacks is found in the 

 larvae of almost all Gasteropods and Pteropods, and usually 

 originates very early. They are placed in the front part of the 

 foot, and on the formation of the pedal ganglia come into close 

 connection with it, though they receive their nervous supply in 

 the adult from the supra-cesophageal ganglia. 



In a very considerable number of cases amongst Gasteropods 

 and Pteropods the auditory organs have been observed to develop 

 as invaginations of the epiblast, which give rise to closed vesicles 

 lying in the foot, e.g. Paludina, Nassa, Heteropods, Limax, some 

 Pteropods (Clio). 



This is no doubt the primitive mode of origin, but in other 

 cases, which perhaps require confirmation, the sacks are stated 

 to originate from a differentiation of solid thickenings of the epi- 

 dermis or of the tissues subjacent to it. 



The auditory sacks are provided with an otolith, which 

 according to Fol's observations is first formed in the wall of the 

 sack. 



In Cephalopods the auditory organs are formed as epiblastic 

 pits on the posterior surface of the embryo, and are at first 

 widely separated (fig. 113, ac). The openings of the pits become 

 narrowed, and finally the original pits form small sacks lined 

 by an epithelium, and communicating with the exterior by 

 narrow ducts, equivalent to the recessus vestibuli of Vertebrates, 

 and named, after their discoverer, Kolliker's ducts. The ex- 

 ternal openings of these ducts become completely closed at 

 about the same time as the shell-gland, and the ducts remain as 

 ciliated diverticula of the auditory pits. The widely separated 

 auditory sacks gradually approach in the middle ventral line, 

 and are immediately invested by the visceral ganglia (fig. 124, 

 ac). They finally come to lie in contact on the inner side of 

 the funnel. 



On the side opposite Kolliker's duct, an epithelial ridge is 

 formed the crista acustica the cells of which give rise to an 

 otolith connected with the crista by a granular material. At a 

 later period of development three regions of the epithelium of 

 the sack become especially differentiated. Each of these regions 



