. Biology; the story of living things. SUPPORT, MOTION, AND SENSATION 361 Ears The structures making up the compUcated mechanism of iiearing primarily serve two purposes, namely equilibration and hearing. Of these functions the first is luidoubtedly the more primitive. Most invertebrates, whether jellyfish, molluscs, or crayfish, main- tain their equilibrium by some sort of otocyst. Roughly described, this consists of a sac lined throughout or in part by cell-receptors and containing concretions called otoliths. As the animal changes its position the otolith shifts due to the forces of gravity
. Biology; the story of living things. SUPPORT, MOTION, AND SENSATION 361 Ears The structures making up the compUcated mechanism of iiearing primarily serve two purposes, namely equilibration and hearing. Of these functions the first is luidoubtedly the more primitive. Most invertebrates, whether jellyfish, molluscs, or crayfish, main- tain their equilibrium by some sort of otocyst. Roughly described, this consists of a sac lined throughout or in part by cell-receptors and containing concretions called otoliths. As the animal changes its position the otolith shifts due to the forces of gravity and thus stimu- lates by contact the different receptor nerve cells, which transmit the impulse of pressure to the ^ ^ endolymphatic Sac endolymphcctic duct anter-ior Semicircular-/ CxxnaL. posterior- Semicircular- Corjal utriculLcs brain, where it is interpreted so as to enable the animal to right itself. The ecjuilibratory mecha- nism of vertebrates functions principally through stimuli received from nerve cells located in the arnqmllae or swollen ends of three semi- circular canals, occupying roughly the three planes of space. The animal is enabled to adjust its position wdth reference to the stimuli re- ceived through the influence of gravity. In such cases the fluid within the semicircular system stimulates differen- tially the nerve endings in the ampullae. Stimuli reach the nerve-receptors in the same manner as they do in the lower forms, being carried by branches of the auditory nerve (VIII) in the brain. The entire structure is protected by a surrounding mass of cartilage which in higher forms becomes ossified. As to the function of hearing, it is possible that in the case of fishes vibrations are transmitted by the water through the skull to the sensory inner ear. However, when air is substituted for water as the chief environment some other more sensitive device must be ' Semicirculctr- Cocnccl The inner ear of a fish showing the essential features of th
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