Hearing, and the Ear. 465 



hairs. Other medusae have auditory organs in which the vesicle is 

 closed and the otoliths reduced in number. In others the organ is a 

 modified tentacle which is, in some cases, more or less completely en- 

 closed in a cup-shaped cavity. The organ is supplied with . otoliths 

 situated at its apex. (Lubbock "Senses of animals.") In the crabs, 

 lobsters and other crustaceans, the hearing organ is furnished with auditory 

 hairs which are connected with the nerve. In most cases the ear is in 

 the head at the base of the small antennae, but in the Mysis, a genus 

 resembling the Shrimp, the ears are in the tail. Hensen found that the 

 auditory hairs of different length vibrated under the influence of sounds 

 of different pitch. He < ' took a mysis and fixed it in such a position 

 that he could watch particular hairs with a microscope. He then 

 sounded a scale ; to most of the notes the hair remained entirely passive, 

 but to some one it responded so violently and vibrated so rapidly as to 

 become invisible. When the note ceased the hair became quiet, as soon 

 as it was resounded the hair at once began to vibrate again. Other hairs 

 in the same way responded to other notes. " 'The vibration of the hairs 

 is mechanical and does not depend on the animal being alive. (Lubbock.) 

 The ear of the crustacean Cray-fish, is placed back of the eyes at 

 the root of a pair of short branched antennae. There is a cavity with a 

 small entrance on top, surrounded by hairs, and closed by a mem- 

 brane. Inside the cavity and nearly filling it is a sac nearly filled with 

 fluid. In the bottom part of the sac is a little ridge ( crista acustica ) 

 from which sprout a number of vertical hairs. Fine white threads, 

 fibres from the auditory nerve, ramify on the ridge and communicate 

 with the roots of the hairs. In comparing this simple auditory organ 

 with the human ear the sac appears to correspond with our vestibular 

 sac, the ridge supporting the hairs to our crista acustica. The auditory 

 fibres begin, in both cases, at the roots of the hairs on the ridge. The 

 mechanical action of the sense organ terminates in the same place in 

 both cases and surrenders the stimulus to the afferent nerves. This 

 point is the essential part of the ear in all cases. The additions made 

 by long habit and selection to the ear of our inheritance the semi- 

 circular canals with their ampullae, the cochlea, with its three-story 

 stairway, the basilar membrane with its keyboard of Cortis arches, the 

 drum with its bony sounding post, the auditory canal and the ear flap, 

 are so many improvements, more or less u modern " by which new and 

 various properties of sound agitations have succeeded in making them- 

 selves felt by us, or the old properties more perfectly emphasized. 

 Among the mollusks the ears are of a simple character. That of the 

 Unio is shown in fig. 205. Simple as it is, the stone, the hairs, the 

 cells and the nerve are present. Fig. 206 shows a little more perfect 

 arrangement, as the ear of a free swimming Grasteropod, the Pterotrachea. 



