SENSE ORGANS. 289 



which is a persistent relic of the involution of the 

 dorsal integument from which the ear is developed. 



Pass a seeker down the aquceductus vestibuli into the 

 vestibule. 



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 

 receives 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 ampullae 

 . of the semicircular canals. 



D. The Sensory and Ampullary 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 ampullae are collected principally into a large mass 

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



