SPECIAL ORGANS. 99 



* 



chain of bones, it is brought in connection with the swim 

 bladder, so that by this arrangement the intensity of the 

 vibrations of sound is augmented. In other osseous fishes, 

 as in Cottus, the swim bladder extends, without such a chain 

 of bones, to membranous spaces in cranium which are in 

 connection with the auditory apparatus. (Van der Iloeven.) 



Batrachia. Internal ear based upon same plan as in Pisces; 

 vestibule, however, is proportionately smaller. In Proteus 

 (perenuibranchiate) it is imperfectly, but in Rana (caduci- 

 branchiate) completely embedded in otic bones. Otolithes 

 granular. Middle ear absent in aquatic types, as in l^roteus; 

 rudimentary in others. Generally remote, the mouths of 

 Eustachian tubes sometimes unite in middle of palate. Tym- 

 panic ossicle (columnella) single. Tympanic membrane on 

 side of head in Sana. 



Reptilia. Internal ear completely enclosed within otic 

 bones. Cochlea present, rudimentary, slightly curved, not 

 divided by internal lamina (scala) except in Crocodilia. Oto- 

 lithes in Ophidia dense, in Chelonia and Sauria softer. 

 Middle ear larger in Sauria than Ophidia; traversed by 

 columnella, and covered externally by a thin, transparent, 

 naked membranous continuation of the skin placed on a 

 level with general surface of head, or slightly projected, as in 

 Crocodilia. (Grant.) In Ophidia the tympanic membrane is 

 covered by skin and muscles. 



Aves. Internal ear with narrow vestibule ; cochlea curved, 

 and, assuming the form of the spiral, is divided internally 

 by lamina. Semicircular canals long and narrow in ra- 

 pacious birds, large and wide in singing birds. Otolithes 

 small. Middle ear traversed by columnella and vidian nerve. 

 Eustachian tubes commonly unite before they open into pos- 

 terior nares. Walls of cavity pierced by several foramina 

 which lead to large cells between two tables of skull. Ex- 

 ternal ear present in most birds in the form of a crescentic 

 fold of skin extending upward from the superior margin of 

 the external meatus. (Grant.) 



Mammalia. Internal ear with vestibule comparatively 

 small. Cochlea, excepting in Ondthorhynchus (duck-bill), 



