280 THE DOG-FISH. 



The spherical shape of the lens, together with 

 the flatness of the cornea, is very characteristic of 

 aquatic animals. 



C. The Ear. 



The ear consists of a membranous sac, or vestibule, filled 

 with fluid, and produced into three curved tubes, the semi- 

 circular canals, which lie in planes of which each is at right 

 angles to the other two. Each canal opens at both its ends 

 into the vestibule. 



The whole sac is lodged within the cartilaginous auditory 

 capsule, which is firmly united with the side of the hinder end 

 of the skull. 



Scrape clean the hinder end of the shull, so as to expose the 

 left auditory capsule fully. Note, on the dorsal surface, the 

 prominent ridges lodging the anterior and posterior vertical 

 semicircular canals, the former running backwards and in- 

 wards from the hinder margin of the orbit, and the latter 

 bachwards and outwards to the posterior end of the skull. 

 Note also, at the inner side of the point of meeting of the two 

 canals, a hole leading downwards and outwards into a canal, 

 the aquaeductus vestibuli. 



Hold the skull in your left hand and carefully pare away 

 the cartilage with a sharp scalpel until the several parts of 

 the auditory organ are fully exposed. Owing to the trans- 

 parency of the cartilage, the parts can be seen before they are 

 met with, and the dissection, though requiring care, is not 

 difficult. 



1. The vestibule is a laterally compressed sac, lying in the 

 lower and inner part of the auditory capsule. It is 

 partially divided into two ; contains milky calcareous 

 concretions ; and communicates with the dorsal sur- 

 face of the head by the aquaeductus vestibuli, which 

 is a persistent relic of the involution of the dorsal 

 integument from which the ear is developed. 



Pass a seeker doivn the aquceductus vestibuli into the 

 vestibule. 



