THE SENSE ORGANS 



597 



The cavity of the otic capsule in amphibia becomes closed and inde- 

 pendent of that of the cranium. Near the lagena, another outgrowth of 

 the sacculus, the basilar papilla, is formed. From this and the lagena, 

 arises the cochlea of the higher vertebrates. To its static function, the 

 ear of the amphibia now adds an auditory one. (Fig. 494) 



The loss of gills in land amphibia is associated with important changes 

 in the visceral arches. The hyomandibular cartilage ceases to be a sus- 

 pensory apparatus of the jaw, and slips into the spiracular passage as the 

 stapes or columella. Amphibia are, therefore, the first animals to add to 



Fig. 494. — The left membranous labyrinth of vertebrates as seen in lateral view. 

 A, Myxine; B, Petromyzon; C, Teleost; D, Prog; E, Crocodile; F, Bird; G, Mammal. 

 The series represents fairly closely an evolutionary series. (Redrawn after Hesse.) 



the inner membranous ear structures corresponding to the middle ear of 

 mammals. (Fig. 501) 



In reptiles the lagena and basilar papilla unite in an elongated cochlea, 

 which in crocodiles becomes spirally wound. By the attachment of the 

 cochlear duct to the bony labyrinth along two sides, the perilymphatic 

 cavity is divided into two portions, the scala vestibuli and the scala 

 tympani. Further advance towards the mammalian ear is seen in the 

 appearance of a membrane-covered window, the fenestra vestibuli, to 

 which the stapes is attached. As a result of these advances, the hearing 

 of reptiles is noticeably keener than that of creatures lower in the scale. 



In mammals, the length of the cochlea varies from a half-turn in 

 echidna to three and a half turns in the deer. The keenness and range 

 of the sense of hearing differ correspondingly in the two animals. In 

 correlation with the elongation of the cochlea, the length of Corti's organ 



