2l6 



BONY FISHES 



vii. 14- 



cornea is thickened, the iris provides two pupils, the lens is pear- 

 shaped, and there are two retinas in each eye. 



15. Ear and hearing of fishes 



The ear provides receptors that ensure the maintenance of a correct 

 position of the fish in relation to gravity and to angular accelerations. 

 In addition, in many species it serves for hearing. The inner ear is 

 completely enclosed in the otic bones. There is a perilymphatic space 

 only in those species that hear well. 



ant. 



Ns Sag. S. L. 



Fig. 134. Diagram of ear of the minnow Phoxinus. 



Ast. asteriscus; L. lagena; N.I., N.s. nerves of lagena and saccule; 



S. saccule; Sag. sagitta; U. utricle. (From V. Frisch, Z. vergl. 



Physiol. 25.) 



Each ear sac is subdivided into three semicircular canals and three 

 other chambers, the utriculus, sacculus, and lagena (Fig. 134). In 

 each chamber is carried an ear stone (otolith) and these are given 

 special names, the lapillus, sagitta, and asteriscus, occupying the above 

 three chambers respectively. 



The sensitive macula of the utricle lies horizontally, with the 

 lapillus resting upon it, whereas the maculae of the saccule and lagena 

 are vertical. These receptors with otoliths have double or triple 

 functions. At rest they act as static receptors, signalling the position 

 of the fish in relation to gravity and setting the fins and eyes in appro- 

 priate positions. In movement, together with the semicircular canals, 

 they signal angular accelerations, initiating compensatory movements. 

 Thirdly, some of the otolith organs respond to sonic vibrations. 



In the fishes that hear well there is a connexion between the air 

 bladder and the ear. This may be either direct, by means of a sac 

 extending forwards (in Clupeidae and others) or indirectly by a chain 

 of modified vertebrae, the Weberian ossicles (Fig. 135). This latter 

 arrangement is found in the freshwater Ostariophysi, which hear 



