

380 MANUAL OF ZOOLOGY SECT. 



the eyes, and the auditory organs. The olfactory organs are a 

 pair of cup-like sacs on the under side of the snout, enclosed 

 in the olfactory capsules and opening externally by the 

 nostrils. They are lined with mucous membrane, which is 

 raised up into ridges so as to increase the surface. The 

 general structure of the eyes has already been described 

 (p. 354). The ear consists of the membranous labyrinth 

 (p. 356), which is enclosed in the cartilage of the auditory 

 region of the skull. It consists of a sac called the vestibule 

 (Fig. 212, vest.), with which are connected three tubes, 

 called from their form the semicircular canals. Two of these, 

 the anterior and posterior canals, are vertical in position, 

 and are united with one another at their adjacent ends ; at 

 the other end each is dilated to form a bulb-like swelling, 

 the ampulla. The third, or horizontal canal (hor. can.) 

 opens at each end into the vestibule, and has an ampulla at 

 its anterior end. The vestibule gives off a tube, the 

 endolymphatic duct, which opens at the auditory aperture 

 already referred to on the top of the head. Endolymph 

 containing otoliths (p. 356) fills the interior of the labyrinth, 

 which is immediately surrounded externally by a space con- 

 taining a similar watery fluid, the perilymph. The fibres of 

 the auditory nerve are distributed to various parts of the 

 internal epithelium of the vestibule and semicircular canals. 

 There seems little doubt that the membranous labyrinth 

 has not only an auditory, but also an equilibrating function, 

 i.e., that the fish is enabled by its means to maintain its 

 equilibrium in the water. 



The kidneys (Fig. 202, kd.) are long flat lobulated 

 bodies lying one on each side of the backbone in the 

 posterior part of the abdominal cavity. From the ventral 

 surface of each spring numerous delicate ducts which open 

 into a single wide tube, the ureter. In the female, the 



