METABOLISM OF FOODS AND DISPOSAL OF WASTES 



521 



mal gives some clue as to the nature of its 

 diet. 



The remaining portion of the original 

 amino acid is finally either burned com- 

 pletely to COo and HoO, delivering large 

 amounts of energy, or is converted to glu- 

 cose which eventually oxidizes in the usual 

 manner. 



The metabolism of the animal is destiuc- 

 tive for the most part. Of course, most of 

 the energy is expended in the mere business 

 of keeping alive — moving about, producing 

 heat, ingesting, digesting, distributing food, 

 and disposing of wastes. But even the syn- 

 thesis of fats, glycogen, and proteins depends 

 on energy derived from the destructive 

 forces in the cell. And, as the animal in- 

 creases in size, a tremendous amount of 

 organic matter must be torn down in order 

 to synthesize sufficient protoplasm to pro- 

 vide for growth. So the cycle goes on. 

 What the plants build up, the animals break 

 down, each complementing the other. It 

 had to be thus or there would have been no 

 animals, and had there been no animals, the 

 world would soon have been overrun with 

 plant products which would have tied up 

 all of the available COo and other critical 

 compounds in the form of organic mole- 

 cules. 



DISPOSAL OF WASTES 



Once the food products are metabolized, 

 the resulting waste products must be 

 promptly removed from the body. Retained, 

 they act as toxic substances, causing death 

 of the animal within a short time. The 

 elimination of these wastes is very simple in 

 the unicellular animals like amoeba (Fig. 

 20-1) and even in multicellular ones like 

 hydra. Indeed, wherever all or even most 

 of the cells are still in contact with water in 

 the external world, there is no problem, for 

 the CO2 and nitrogenous wastes diffuse 

 through the cell membranes into the sur- 

 rounding water. When many of the cells 

 of the animal body lie deeper, the elimina- 



tion of metabolic wastes requires special 

 organs. This has resulted in the formation 

 of the excretory systems: gills or lungs for 

 the removal of COo, and kidneys for the 

 removal of nitrogenous wastes. Sweat glands 

 in some mammals also aid in excretion. 



These systems have all been described 

 in preceding chapters, and we shall here 

 confine the discussion to a brief account of 

 the major steps in the evolution of kidneys. 

 We have seen that the first simple kidney 

 consists of a system of tubules ramifying 

 the body tissues, as illustrated in planaria 

 (Fig. 20-1). Each tubule drains the tiny 

 flame cells which selectively pick up the 

 nitrogenous wastes from neighboring cells, 

 and the entire system conveys those prod- 

 ucts to the outside through many pores. 



The next great step is taken by the anne- 

 lids, where nephridia replace the flame 

 cells. Not only do the cells at the funnel 

 (nephrostome) pick up coelomic fluid 

 which contains many substances besides 

 nitrogenous wastes, but as this fluid passes 

 down the tubule selective reabsorption 

 occurs along the way. This principle estab- 

 lished in the annelids is retained through- 

 out all higher groups. 



In the vertebrates the origin of the ex- 

 cretory system is intimately associated with 

 the reproductive system. Primitive cyclo- 

 stomes have kidneys that are not greatly 

 different from those found among inverte- 

 brates. They are long, thin, paired struc- 

 tures lying in the dorsal wall of the body 

 cavity, one on each side of the vertebral 

 column (Fig. 20-2). Coelomic fluid is 

 drawn into the ciliated nephrostomes which 

 lead into a long; tubule much like the earth- 

 worm. The tiny tubules coalesce forming 

 larger tubes, the urinary ducts, which ter- 

 minate in the cloaca. In this animal, the 

 eggs and sperms are shed into the body 

 cavity and find their way out through two 

 openings from the posterior end of this cav- 

 ity into the cloaca. This is an extremely 

 simple method reminiscent of some of the 

 lower invertebrates. The excretory system 



