144 Mammalia. Organ of Hearing. 



Of all parts of the Labyrinth the Cochlea varies most; 

 m., in the number of its coils, which may present all 

 degrees of complexity, from half 2^ turn to nearly ^i?tf complete 

 turns. 



While in Man the muscles of the external ear are only 

 feebly developed, so that it can be but slightly moved, 

 very numerous muscles turn the Ear of the lower Mammalia 

 in all directions. In many diving animals peculiar valve- 

 shaped projections are found, by which the external meatus 

 is closed and protected against the entrance of water. 



Special. 



1 . The Concha is absent or rudimentary 



in the 7a/j!?/'</<s(Insectivora); in the Phocidce^TA Tnch(ecid(B{^eQ' 

 torialia Pinnigrada) : in the CeteivfYiero, in a full-sized Cachalot, the 

 external opening of the ear is a longitudinal slit, one inch in length, 

 admitting with difficulty the end of the fore-finger ; and in the com- 

 mon Porpoise and Dolphin is so small as to require search in detect- 

 ing) : in the Sloth (rudimeutal) and Mams (scale-like) of the In- 

 Enamellata : and in the MoJiotremata. (In the Echidna the external 

 aperture presents the form of a vertical slit, shaped like the italic aS, 

 an inch and a half in length.) 



2. Meatus externus. 



In Echidna the remarkably long Meatus is strengthened by a 

 series of incomplete cartilaginous hoops, connected together by a 

 narrow longitudinal cartilaginous band, so that its structure closely 

 resembles that of a trachea. The narrow tortuous meatus of Orni- 

 thorhyncus has a valve externally. 



In the Water Shrew the antitragus can be brought forward so as 

 to close the external meatus at will. 



3. Malleus. 



Bird-like : e.g. Perameles (Marsupialia). 



Lamelliform : e.g. Shrews (Insectivora). 



Circular: e.g. in the Armadillos (In-Enamellata) where the 

 Malleus is bent, almost, as in the Monotremata, completing the circle 

 for the drum membrane. 



Perforated : e.g. in the "Wolverine (Plantigrada ; Guh^ and Otter 



