SENSE ORGANS. 



75 



brane is placed at the bottom of a tube, the external auditory 

 meatus, the outer end of which is frequently protected by 

 movable dermal flaps. In most mammals an external ear, sup- 

 ported by cartilages, is developed ; and there is considerable 

 evidence to show that this external ear is a derivative from the 

 operculum of fishes, or from the external branchial structures 

 of the amphibia. 



Recent experiments tend to show that in the fishes the ears 

 are without auditory functions and are solely organs of equili- 

 bration. In terrestrial vertebrates they are both organs for 

 hearing and for the maintenance of the equilibrium. 



Olfactory Organ. The organ of smell is a single sac in the 

 cyclostomes, paired in all other vertebrates. Its essential por- 

 tion is the sensory epithelium, in 

 which sensory cells are inter- 

 spersed with supporting or isolat- 

 ing cells. Its nerve supply is 

 the olfactory nerve already de- 

 scribed. The powers of smell 

 are directly proportional to the 

 extent of sensory surface, and in 

 order that this may be increased 

 the surface is folded, usually in 

 the longitudinal direction. In the 

 more primitive forms the sensory 

 surface is not uniformly distrib- 

 uted, but is gathered in patches 

 separated by large masses of iso- 

 lation cells. In some ganoids 

 and amphibians the nasal epithe- 

 lium has a peculiar radiate ap- 

 pearance, as seen in transverse 

 section. From the amphibia up- 

 wards outgrowths of cartilage or bone (turbinals), either from 

 the ethmoid or lateral walls, tend to divide the cavity still 

 further. In the petromyzontes and pisces only external nos- 

 trils occur, and in the cyclostomes there is but one of these. In 

 the forms with paired cavities there is primitively but a single 



FIG. 80. Divided nostrils of fishes ; 

 upper figure Acanthias, showing them 

 divided by a movable flap. Lower, 

 young Amia; n, ', anterior and pos- 

 terior narial openings. 



