80 ELEMENTARY ZOOLOGY. 



into which the digestive tract is differentiated. Compare 

 both of these with the condition in the crayfish or locust. 

 (Fig. 5)- 



84. Practical Exercises for Library. Why do animals need a 

 complex digestive tract any more than plants do? Enumerate 

 the regions in the digestive tract of man (see Human Physiologies). 

 What are the reasons for this .complex differentiation? Why is 

 the digestive tract so long in some animals? What could have 

 served as a cause in bringing about the greater growth of the lining 

 of the digestive tract into folds and elevations? What advantage 

 has these folds to the animal? Is the digestive tract in the crayfish 

 longer than the body itself? What provision does the crayfish 

 have for physical digestion? For chemical? 



85. The Respiratory Organs. Closely connected with 

 the need of food is the need of oxygen, which must be 

 had by all protoplasm. Closely connected with the use 

 of oxygen is the formation of CO 2 , which for the health 

 of the body must be removed from the cell where it is 

 formed and then from the body as a whole. The term 

 respiration is applied to the whole process and consists of 

 the following steps: (i) taking oxygen from the medium 

 (air or water) into the body fluids; (2) carrying O to the 

 system; (3) transfer of O from the body fluids to the cells; 

 (4) union of O with certain materials in the protoplasm 

 forming, among other waste products, CO 2 ; (5) transfer 

 of CO 2 to .the blood; (6) CO 2 carried to the respiratory 

 organ; (7) CO 2 given up from the blood to the outside 

 medium. 



The oxygen may be absorbed and the carbon dioxide 

 eliminated through any moist, thin-walled structure 

 containing blood or other body fluid; but in order to do 

 this rapidly enough for the needs of active animals there 

 must be much of this surface freely exposed to the air or 

 water. This surface is the respiratory organ. 



