416 BIOLOGY. [BOOK vi. 



vesicles, in which float powerfully refracting homogeneous granu- 

 lations. These vesicles receive nerves, and sometimes even rest 

 on the central ganglions of the nervous system. 1 



There is the same structure of the auditory organs in the 

 mollusks. Those organs are always small bags full of liquid and 

 containing otoliths formed of an organic basis, 

 impregnated with calcareous substance. These 

 otoliths sometimes amount to many hundreds 

 (Fig. 75). 



The auditory organs of the arthropods are 

 more varied, and are known only in some 

 FIG. 75. divisions of the crustaceans and of the insects. 



Auditory organ of Cy- In the crustaceans, the auditory vesicles are 

 suie';' & e, epithelial sometimes open. The closed vesicles contain 

 cilia 3 ; otoiith. W1 otolithical concretions fixed by fine hairs regu- 

 larly arranged. According to Hensen, auditory 

 hairs, free, sometimes exist outside of the vesicles. These hairs 

 even enter into vibration, isolately, when musical sounds are 

 produced. Each of these vibrates in the water according to a 

 special musical sound. 



The organs of hearing proceed in the vertebrates from an 

 invagination which is produced from the two sides of the head 

 at the commencement of the embryonary life. This vesicle, at 

 first in extensive communication with the exterior, closes little 

 by little. The most inferior of the vertebrates, the amphioxus, 

 seems destitute of auditory organs. 2 According to Schultze, the 

 auditory organ of the cyclostomes appears to be first of all a 

 vesicle containing a rounded otolith. 3 



Most fishes have as organs of hearing ampullae full of liquid 

 and imperfectly divided by an incomplete partition, on which 

 the nervous terminations are spread. The internal face of the wall 



1 Leydig, toe. cit., p. ai6. 



2 Huxley, Anatomia, Cbmparata dei Vertebrati, p. 71. (Jtalian transla- 

 tion of En. Giglioli.) 



3 Entweilung von Petromyzon Planeri, Harlem, 1 856. ' 





