SENSE ORGANS. 281 



2. The semicircular canals are three in number. They 



open at both ends into the vestibule, and each is 

 dilated at one end into an oval ampulla, which re- 

 ceives a branch of the auditory nerve. 



i. The anterior vertical canal has its ampulla at 

 the anterior end close to the orbit. 



ii. The posterior vertical canal is the longest of 

 the three, and forms an almost complete circle. 

 Its ampulla is at its posterior and inferior end, 

 on the ventral wall of the skull. 



iii. The horizontal canal lies in the wing-like 

 lateral process of the auditory capsule, behind 

 the orbit : its ampulla is at its anterior end, 

 close to that of the anterior vertical canal. 



3. The auditory nerve enters the auditory capsule on its 



inner side, and at once divides into branches, which 

 can be traced to the vestibule and to the ampullfB 

 of the semicircular canals. 



D. The Sensory and AmpuUary Canals. 



The dog-fish possesses at least two kinds of tegumentary 

 sensory organs. (1) The sensory canals, a continuous system^ 

 developed along certain lines on the head, the jaws, and down 

 the sides of the body forming ' the organs of the lateral line.' 

 These structures are found in almost all fishes. (2) The blind 

 and more or less short ampullary canals not found outside the 

 group of Elasmobranchs, and distinguished from the*, sensory 

 canals by the swellings or ampullae at their blind ends. 



These ampullee are collected principally into a large mass 

 at the anterior end of the snout, in front of the cranial cavity ; 

 and two smaller masses at the sides of the head, between the 

 nose and the orbit. From these masses the tubes diverge to 

 their external openings on the dorsal and ventral surfaces of 

 the head. The tubes are filled with a transparent gelatinous 

 matter, which can be squeezed out from their openings. 



The function of the two canal systems is not known with 

 certainty, but they are generally believed to be sense organs 



