COORDINATION IN ANIMALS 



197 



ass 



that equilibration is the chief function of the entire labyrinth 

 in Fishes, as it is of the so-called auditory organs of many 

 Invertebrates, such as the Crayfish. With the progres- 

 sive specialization of the labyrinth, the essential sensory 

 cells, which are in communication with the brain by the 

 eighth, or AUDITORY NERVE, become 

 limited to a few definite areas. 

 These sensory cells are provided 

 with auditory hairs which project 

 into the cavity of the labyrinth and 

 so are stimulated by movements of 

 the fluid which fills it. (Fig. 109.) 



The ears of Fishes lie immediately 

 below the skull roof, where they are 

 readily accessible to vibrations 

 transmitted by the water. But 

 with the substitution of air for water 

 as the surrounding medium, there 

 arises the necessity of a more deli- 

 cate method for conducting and 

 also for collecting and augmenting 

 the sound waves. The result is 

 that, in ascending the Vertebrate 

 series, we find the ear proper receding farther and farther 

 below the surface. 



Soon, between the inner ear and the surface of the head, a 

 simple resonating chamber is added which is provided with a 

 vibrating TYMPANIC membrane, or EAR DRUM, situated just 

 under the skin. Then this is improved by the development 

 of a bony transmitting mechanism between the tympanic 

 membrane and the inner ear. This consists of a single bone 

 until we reach the Mammals, when two more bones are added 

 by being diverted from their earlier function of articulating 



FIG. 109. Semidiagram- 

 matic figure of the left mem- 

 branous labyrinth of a lower 

 Vertebrate to show the sacculus 

 (s, 1), utriculus (u, rec), and the 

 three semicircular canals (aa, ca; 

 ae, ce, and ap, cp). I, lagena, a 

 derivative of the sacculus which 

 becomes the cochlea in higher 

 Vertebrates; cus, utriculo-saccu- 

 lar canal; de, se, endolymphatic 

 duct and sac ; ass, sp, ss, utricular 

 (After Wiedersheim.) 



