xil PHYLUM CHORDATA 383 



hyoid and first branchial arches ; and the tenth, or vagus 

 (vag), to the remaining branchial arches, as well as to the 

 heart, stomach, and lateral line. 



Besides the lateral line, which is probably the seat of a 

 delicate tactile sense, and the tongue, which is presumably 

 an organ of taste, there are the three pairs of characteristic 

 sensory organs, the structure and position of which are very 

 characteristic of vertebrates. These are the olfactory organs, 

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

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

 closed 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. 356). The ear consists of the membranous labyrinth 

 (p. 357), which is enclosed in the cartilage of the auditory 

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

 (Fig. 228, 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. 358) fills the interior of the labyrinth, 

 and it 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 



