METABOLISM AND TRANSPORT IN ANIMALS 



541 



ous and siliceous skeletons of protozoans, 

 the spicules and spongin in sponges, the 

 coral in coral polyps, and the calcareous 

 plates in starfishes. 



3. Digestive glands that secrete digestive 

 juices; a list of those in man is presented on 

 page 517. 



4. Nutritive glands that secrete milk and 

 other products that serve as food for the 

 young, such as the mammary glands of 

 mammals, and those lining the crop of the 

 pigeon that secrete the cheesy substance 

 called pigeon's milk. 



5. Poison glands that secrete poisonous 

 substances for purposes of defense or of- 

 fense, such as those in the skin of frogs, 

 toads, and salamanders, in the sting of the 

 bee and the scorpion, and those located in 

 the heads of the Gila monster and the 

 rattlesnake. 



6. Constructive glands that produce silk 

 with which caterpillars build cocoons and 

 spiders build webs, and wax with which bees 

 construct honeycomb. 



Many other examples of glands could be 

 listed from those we have encountered dur- 

 ing our studies of animals. 



UTILIZATION OF 

 FOOD IN THE BODY 



Now that we have learned what food is, 

 where it is obtained, how it is captured, how 

 it is ingested, how it is digested, how the 

 products of digestion are distributed 

 throughout the body and absorbed, how 

 oxygen is obtained during respiration, how 

 secretions are formed for digestion and for 

 other purposes, and how the wastes in- 

 volved in these processes are eliminated, we 

 are ready to consider the utilization of food 

 in the body. What becomes of oxygen, 

 water, mineral salts, vitamins, carbohydrates, 

 fats, and proteins, and what are the re- 

 quirements of animals with respect to these 

 food substances? 



Oxygen 



Oxygen enters cells from the medium in 

 which the cells live (water in many aquatic 

 animals, and tissue fluid or blood in higher 

 animals); it is used in building up proteins 

 within the cell and in the production of 

 energy by oxidation. 



Water 



Water makes up about 60 to 90 per cent, 

 by weight, of protoplasm. It is ingested in 

 greater amounts than all other substances 

 combined, and it is also an important excre- 

 tion. It is the vehicle of the principal foods 

 and excretion products, for most of these are 

 dissolved as they enter or leave the body. In 

 fact, as clearer ideas of the physicochemical 

 organization of protoplasm have developed, 

 it has become evident that the organism 

 itself is essentially an aqueous solution in 

 which are spread out colloidal substances 

 of great complexity. As a result of these con- 

 ditions, there is hardly a physiologic process 

 in which water is not of fundamental im- 

 portance. 



Mineral salts 



Salts are essential constituents of cells and 

 of the environment in which cells live. They 

 take part in the building up of protoplasm 

 and in the activities of various types of 

 cells. For example, iron is a necessary con- 

 stituent of hemoglobin, iodine of the hor- 

 mone thyroxin of the thyroid gland, and 

 calcium and phosphorus of the bones. In 

 many cases only a minute quantity of a 

 mineral is required, but without this health 

 would be impaired or life impossible. For 

 example, the necessary daily quantity of iron 

 is only 0.006 gram and of iodine only 0.00001 

 gram, but calcium and phosphorus are re- 

 quired in great abundance. 



Carbohydrates 



Carbohvdrates are furnished to the cells 

 in the form of simple sugars. They may com- 



