86 FIRST LESSONS IN ZOOLOGY 



only brings oxygen to the respiratory surface, and before 

 tlie real respiration at the tissue-cell can take place the 

 oxygen must be carried by the blood to the cell. This 

 process we shall later discuss under the head of circulation. 



Now having seen how animals get the necessary oxygen 

 we may next inquire how they obtain and make use of 

 the equally necessary substances to be oxidized and to 

 build the body out of, that is, their food. 



How animals obtain and digest food. — In examining 

 into the ways by which animals obtain and use food we 

 may consider the processes under three heads, obtaining 

 food, eating it, and digesting it. In obtaining food all 

 the sense and motor organs are employed, that is, the 

 instincts and ingenuity of the animal are brought into 

 play as well as the parts of its body. In eating it the 

 mouth is employed in mastication and the throat in swal- 

 lowing. Digestion is carried on by liquids secreted by 

 parts of the alimentary canal. 



In the parts used for obtaining food we necessarily 

 find the greatest range of variation on account of the 

 great variety of food materials made use of. The food 

 of animals consists of other animals or plants. Of the 

 plants it may be leaves, stem, wood or bark, roots or 

 fruit or seeds. That particular part desired may be deep 

 in the ground, high in the air, or buried in a thick cover- 

 ing of its own. The plant sought may grow in a marsh, 

 on a plain, on a mountain, or immersed in the ocean. 

 It may be fit for food at a certain season only. To get 

 it the animal must have the necessary equipment of wings, 

 or legs, claws, beaks, teeth, etc., as well as eyes, ears, and 

 organs of smell, touch, and taste, besides some means for 

 searching out the places where it grows, selecting the 

 proper parts and deciding upon ways of securing these 

 parts. Acts like these last are performed b\' the nervous 

 system and arise from instinct or intelligence. 



