SENSE ORGANS AND WHAT THEY DO 427 



Self-Testing Exercise 



Changes in the (1) of leaves are brought about by a 



structure called the (2). Paramecia react (3) to 



a favorable environment and (4) to an unfavorable one. 



Paramecia can be observed to (o) to the presence of 



(6), (7) and (8). If Paramecia cannot escape 



from (9) conditions, they die. Plants change their positions 



in response to such stimuli as (11), (12), 



(13), and (14). 



PROBLEM HI. WHAT ARE SENSE ORGANS AND WHAT DO 



THEY DO? 



Demonstration 4. To show types of sensory structure in certain 

 animals. 



Materials. Insects with different types of antennae. Crustaceans 

 with antennae and antennules. Grasshoppers, with wings removed to 

 show tympanic membrane. Model of vertebrate eye and ear. Living 

 crickets, earthworms, crayfish, and living goldfish. Food, such as 

 apple or meat. Weak acetic acid. 



Method. Arrange preserved specimens and models so that they 

 may be passed around in class or observed on the demonstration table. 

 Living material should be placed in pans or aquariums where they 

 can be fed, and stimulated with weak acid. 



Note the hairs projecting from the antennae and antennules of the 

 insects and crustaceans. They are sensory in nature. Note in the 

 grasshopper the sensory organ, which receives sound. Study the 

 model of the human ear. Does our ear do more than receive sound? 

 Study the model of the human eye. Compare it with a camera. 



Observe carefully what happens when food, such as a bit of apple, 

 is placed in a dish containing live crickets or earthworms. Note also 

 what happens when crayfish or goldfish are fed meat. How do they 

 become aware of the presence of food ? 



Place some cotton soaked in weak acid close to anterior end of an 

 insect, a worm, and a crayfish. What happens? 



How do animals become aware of food or harmful substances ? 



Sense organs and what they do. Most plants do not react 

 quickly to stimuli, because they have no special sense organs. 

 Nor have the one-celled animals any special part of the cell fitted 

 to receive stimuli. But in animals composed of numerous cells, 

 division of labor soon appears, and we have organs fitted to receive 

 light stimuli (eyes), touch stimuli (tactile hairs, etc.), and sound 



