230 AUDITORY ORGANS. 



spherical form (fig. 125, hi). The condition of the eye, with a closed 

 optic vesicle and the lens projecting into it, is that which is 

 permanent in the majority of Gasteropods (vide fig. 122 B). At 

 about the time when the lens first becomes formed a fold com- 

 posed of epi blast and mesoblast appears round the edge of the 

 optic cup (fig. 124, cc), and gives rise to a structure known in the 

 adult as the iris. Shortly afterwards this becomes more pro- 

 minent (fig. 125, if), and at the same time the layers of cells of the 

 ciliary region in front of the inner segment of the lens become 

 reduced to the condition of mere membranes (fig. 125 B); and in front 

 of them the anterior or outer segment of the lens becomes formed as 

 a cuticular deposit (fig. 125 B, vl). At a still later period a fresh 

 fold of epiblast and mesoblast appears round the eye and gradually 

 constitutes the anterior optic chamber (vide fig. 122 C, Co). In most 

 forms this chamber communicates with the exterior by a small 

 aperture, but in some it is completely closed. The fold itself gives 

 rise to the cornea in front and to the sclerotic at the sides. At a later 

 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 form- 

 ation 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 epidermis 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 dis- 

 coverer, Kolliker's ducts. The external 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. 



