182 



ESSENTIALS OF ZOOLOGY 



are absorbed and the condition of the adult organ is reached. 

 The organ comes to be lodged in a cartilaginous capsule, and 

 it consists of a vestibule, the wide part of the original invagina- 

 tion still connected with the skin, by a ductus endolymphaticus, 

 and three semicircular canals. The canals are each dilated 

 at one end into an ampulla. The posterior canal is peculiar 

 in the skate in that it is circular and communicates with the 

 vestibule by a small opening on its inner face. The vestibule 

 and the small expansion called the utriculus, which receives 

 the outer ends of the anterior and horizontal canals, contain 



Saccus endolymphaticus 

 Ductus endolymphaticus 



Posterior semicircular 

 canal 



Ampulla 



Anterior semicircular 

 canal 



Utr. Vest. 



Horizontal semicircular 

 canal 



FIG. 92. Raia. Auditory organ. 



soft otoliths which entangle and are supported by the sensory 

 processes of the cells of these parts of the organ. The ampullae 

 similarly are provided with sensory cells, the processes of which 

 support a gelatinous secretion. The canals are concerned by 

 the flowing of the fluid or endolymph in conveying impressions 

 of changes in position and equilibrium, but there is no doubt 

 that the organ is also able to receive and transmit wave move- 

 ments ; in other words, wave movements which correspond 

 in water to the air movements or sound waves in terrestrial 

 conditions. The endolymph is put into movement, and the 

 movements are communicated by the movements of the otoliths 

 to the sensory processes. 



The auditory organ is a special organ associated with what 

 is called the lateral line system, here in the skate mainly repre- 

 sented by the system of sensory canals, and the system is 



