HEARING. 



569 



municate vibrations ia the invertebrates. When the cephal- 

 opod moUusks, with their highly developed nervous system, 

 are reached, we find a membranous and cartilaginous labyrinth. 

 Among vertebrates the different parts of the mammalian 

 ear are found in aU stages of development. The outer ear may 

 be wholly wanting, as in the frog, or it may exist as a meatus 

 only, as in birds. The tympanic cavity is wanting in snakes. 

 Most fishes have a utricle and three semicircular canals, but some 



Ftg. 415.— Auditory vesicle of a heteropod mollask (Pterotrachea) (after ClauB). ilT, 

 auditory nerve; Oi, otolith in fluid of vesicle; Wz, ciliated cells on inner wall of 

 vesicle; Hz, auditory cells; Cz, central cells. 



have only one ; and the lowest of this group have an ear not 

 greatly removed from the invertebrate type, as may be seen in 

 the lamprey, which has a saccule with auditory hairs and oto- 

 liths, in communication with two semicircular canals. Most 

 of the amphibia are without a membrana tympani. The frog 

 has (1) a membrana tympani communicating with the inner ear 

 by (2) a bony and cartilaginous lever (columella auris), and (3) 

 an inner ear consisting of three semicircular canals, a saccule 

 and utricle containing many otoliths, and a small dilatation of 

 the vestibule, which may indicate an undeveloped cochlea. 

 The membranous labyrinth is contained in a periotic capsule, 

 partly bony and partly cartilaginous, which is supplied with 



