82 ELEMENTARY ZOOLOGY. 



These organs only get the oxygen into the blood. It 

 must then be carried by the circulation to the places 

 where it is really used. The last step, which takes place 

 in the protoplasm itself, is the real respiration. 



The same organs that admit the oxygen allow the CO 2 

 to pass from the blood into the air or water. This is 

 an economy of organs, for the outgo of CO 2 does not in 

 any way interfere with the income of O. 



86. Practical Exercises for the Library. Why do we call the 



respiratory structures of the crayfish gills rather than lungs? Make 

 a diagram showing the relation between the blood and the organ and 

 the water in this case. Does the O used by water animals come 

 from the H 2 O itself? Could you devise an experiment to test 

 this ? Find what variety you can in the location of the respiratory 

 organs in animals. Is there any general law which expresses their 

 position? Describe all the organs in man that take any part in 

 our respiration. 



87. The Circulatory Organs. Because both the food 

 and the oxygen are taken in at special regions, and yet 

 are needed by groups of cells far removed from these 

 points, some device is necessary to carry the materials 

 back and forth. It is clear that this work must be done 

 by a fluid. These body fluids blood and lymph thus 

 render a service to the body similar to that of railroads 

 in a modern community. The larger and more complex 

 the animal, the more necessary and the more difficult the 

 circulation. 



In order that the fluids may be circulated satisfac- 

 torily there must be a series of vessels making a complete 

 circuit; somewhere along the route there must be por- 

 tions of the vessels that can contract in much a way as 

 to force the fluid onward (hearts); in order that the fluids 

 shall travel in a somewhat definite direction there will 



