THE HIGHER ORGANISMS 125 



Gradually the glandular organs are perfected and 

 separate enzymes for acting upon proteins, fats and 

 carbohydrates provided. 



Thus the digestive function ceases to be a cellular 

 function though it continues to be the result of the 

 combined activity of many variously specialized cells. 



The digestive organs also gradually come to stand in 

 close relation with the organs of excretion so that offen- 

 sive digestive products may be removed from the blood 

 before it is distributed to the general system. For this 

 purpose the liver is interposed between the digestive 

 organs and the systemic circulation. 



Vegetables that nourish themselves upon compara- 

 tively simple inorganic compounds diffused through the 

 air and water need no organs of digestion while to 

 animals that nourish themselves upon highly complex 

 vegetable and animal proteins they are indispensable. 



The digestive apparatus thus becomes a factor of 

 much importance in animal morphology and develop- 

 ment. Except among the protozoa and a few para- 

 sitic worms its presence is invariable. It must be 

 proportioned to the requirements of the animal, but 

 just as the animal cannot live without it, it cannot be 

 of use unless means are furnished for providing it with 

 material to work upon. Such material is usually ac- 

 quired through movements effected by special organs. 



The Motor System. Here we have to consider those 

 organs whose primary purpose seems to be to enable the 

 organism to secure its food. The vegetable world is with 

 few exceptions without organs of prehension, motion, 

 or locomotion because they are not needed. Moisture 

 sucked from the soil by roots, and gases and moisture 

 taken from the air by leaves constitute the materials 

 upon which the vegetable world subsists, and as these 

 are always available no special organs are required to 

 obtain them. 



Animal organisms, however, must have food in the 

 form of highly combined products, only to be derived 



