ORGANS OF HEARING. 227 



largest, corresponds to the malleus, and is appended by an unciform 

 process to the swimming-bladder, the middle to the incus, the ante- 

 rior to the stapes. The last-named ossicle may close the prevesti- 

 bulum (atrium sinus imparts), and by pressure upon the swimming- 

 bladder may be drawn apart from or pressed against it. Each pre- 

 vestibulum is provided with a peculiar ossicle encircling it like a 

 staple (claustrum). The auditory ossicles are found distinct in all 

 the species of Cyprinus, in Cobitis, and Silurus, while in other Os- 

 seous Fishes they are wanting. The swimming-bladder is slit in 

 front to form two ducts, which enter the vestibule. 



As Fishes, at least the Osseous, are destitute of external auditory 

 passages, sound in these animals must penetrate or be transmitted 

 through the bones of the head to the labyrinth. The different mem- 

 branous regions of the skull may indeed serve the function of tym- 

 panic membranes, and the swimming-bladder exert some similar 

 office by exercising compression upon the fluid contained in the laby- 

 rinth. 



The organ of hearing becomes remarkably simplified in the lower 

 organized Fishes, and in this respect the Cyclostomi exhibit very in- 

 teresting stages of development, which partly correspond to the fetal 

 structure of the auditory organs in the higher Vertebrata. 



The organ of hearing in Petromyzon and Ammoccetes consists of 

 a bony or cartilaginous part, and of a pair of hard, yellow, oval cap- 

 sules connected with the skull, and enclosing, like a bony labyrinth, 

 the membrane lining the same ; between the two is placed a fibro- 

 membranous layer. The membrane of the labyrinth consists of a 

 small sac, which is divided into two symmetrical cells by an external 

 groove that forms a fold projecting into its interior. Two wide, de- 

 pressed, simicircular canals arise with ampulliform enlargements, and 

 unite into a common opening which enters the vestibule. A single 

 smaller, more rounded appendage to the vestibule corresponds to the 

 auditory sac (sacculus) of other Fishes. Two branches of the audi- 

 tory nerve pass to the supply of the ampullae. 



The structure of the auditory apparatus is still more simple in 

 Myxine and Bdellostoma. It is situated, as in Petromyzon and Am- 

 moco3tes, in a hard ellipsoid capsule, the cavity of which resem- 

 bles a ring, filled up by a similarly formed membranous labyrinth, 

 within which the single semicircular canal is blended with the vesti- 

 bule. 



Otoliths are completely absent in the hearing organs of the Cy- 

 clostomi. Not a trace even of calcareous crystals can be detected 



