416 BIOLOGY FOR BEGINNERS 



different places on the back of the hand with a pencil point, some 

 spots will feel warm and others cold, due to the presence or absence 

 of these temperature nerves. 



Taste. All animals seem to prefer some foods and reject others. 

 We have to assume a sort of taste sense to account for this. To be 

 tasted, a substance has to be in solution and in contact with certain 

 organs near the mouth. The mouth parts, palpi and tongue are 

 the usual taste organs, and in man the different parts of the tongue 

 are sensitive to different tastes. The back part responds only to 

 bitter, the tip to sweet, the sides to sour, and the whole surface to 

 salty flavors. Much that we attribute to taste is really due to the 

 sense of smell; if eyes and nose are closed one can hardly dis- 

 tinguish between an apple, onion, or raw potato. Taste enables 

 animals to judge of foods, stimulates the flow of digestive fluids, 

 and in aquatic forms may give information as to their location 

 in the water. 



Smell. Both touch and taste require the substance to be in 

 actual contact if it is perceived. Smell reaches a little farther 

 away and enables animals to detect substances in the form of 

 vapor or dilute solution, even though at a distance. 



The organs of smell are sometimes hairs, often antennae, while 

 vertebrates have some sort of a " nose." They are usually near 

 the food-getting organs, and in air breathers, are associated with 

 the inlet to the lungs. Primarily the sense of smell is used to judge 

 of food and air supply but in many cases it is also useful in finding 

 food, detecting enemies, and locating mates. It is little developed 

 in aquatic animals but very keen in insects, carnivora, and most 

 ungulates. 



Hearing. In contrast to the three senses mentioned above, 

 hearing puts us in touch with our surroundings through the me- 

 dium of sound waves conveyed by air or water. This brings within 

 range of our consciousness things at a much greater distance and 

 is the chief avenue of communication among all higher animals, 

 most of which possess some form of sound-producing organs. 



The simplest ears in worms, molluscs, and crustaceans consist 

 of mere sacs lined with nerve endings. In insects the sacs are 



