52 



BRAIN AND BODY OF FISH 



vesicle (Fig. iv), about the size of a small pea in its longest diameter, 

 which hes in a vertical plane ; this contains gas ; attached to its 

 base is an almond-shaped sac, containing an otolith ; leading from 

 this and connected to it by a short duct, is a round sac the diameter 

 of which is less than the long diameter of the other sac ; this also 

 contains an otolith. In the floor of the cranial cavity anterior to 

 the air-vesicle are three impressions which lodge the utricle and two 

 ampullse, the spherical terminations of the semicircular canals ; and 

 posterior to the vesicle is another impression for the ampulla of the 





Fig. iv. — Air vesicle, ampullae and canals of Mormyrus — enlarged. 



A.V. — Auditory vesicle, p.s.c.c, h.s.c.c. and a.s.c.c. — Poserior, horizontal and 

 anterior semi-circular canals. S. — Saccule, lag. — Round sac with otoliths. 

 U. — Utride. a. — Ampullae. 



horizontal semicircular canal. These canals embrace the vesicle, 

 but do not communicate with it. 



The almond-shaped sac must be regarded as the saccule, and 

 the round sac presumably represents the lagena, which is the term 

 applied to a specialised portion of the saccule, which is supposed to 

 represent the cochlea of higher vertebrates. This is the nearest 

 approach to the ear of an air-breathing vertebrate that is known in 

 fish. It is true there is no external ear, but there is a tympanic 

 membrane communicating chrectly with a middle ear represented 

 by the air- vesicle, and this is in dii'ect contact with the internal ear, 

 represented by the saccule and lagena and their otoliths. There is 

 another interesting fact that must be noted ; projecting from the 



