SENSES OF ANIMALS 307 



is probaHe that a few birds and also fish use the sense 

 of smell in seeking food. The special end organs of this 

 sense in the vertebrates is located in the mucous membrane 

 lining the upper parts of the nasal cavity, whence a pair 

 of nerves convey the stimuli to the brain. 



With the exception of arthropods, but little is known 

 concerning the sense of smell among the invertebrates. 

 Many insects have a keen sense of smell. A piece of decay- 

 ing meat so covered that it can not be seen will, on a warm 

 day, attract numerous flies, some of which apparently scent 

 it at a, distance where no odor is discernible to man. Many 

 of the social insects recognize the members of their house- 

 hold by smell, and the male of some species seeks his mate 

 by the olfactory sense. Among insects the organs of smell 

 are located in the antennae or palps. Male bumblebees 

 with the antennae removed can not find their mates (Fig. 

 344). Some crustaceans seem to possess organs of smell 

 on the antennules. 



The sense of hearing is most acute in mammals and birds. 

 The organ of hearing, which is a portion of the internal 

 ear located on either side of the head, responds to certain 

 vibrations productive of sound waves in the atmosphere. 

 It is probable that all normal vertebrates with the excep- 

 tion of some fish and certain tailed amphibians are capable 

 of hearing. In the frog, the large round tympanic mem- 

 brane back of the eye (Fig. 239) is similar to the tympanic 

 membrane of birds and mammals located at the inner end 

 of a short canal on either side of the head. It transmits 

 the vibrations affecting it to the inner ear, which is an 

 irregular sac filled with liquid, and containing the termina- 

 tions of the nerve of hearing. No tympanic membrane 



