82 



THE ORGANIZED ANIMAL 



food by the cooperative effort of many cells. 

 The food is captured by the tentacles and 

 taken into the coelenteron where it can be 

 retained until the linino; cells secrete en- 

 zymes to digest it. The end products then 

 can be absorbed directly. 



The cells of the complex metazoan are 

 confronted with the same difficulty in ob- 

 taining food as they were in receiving 

 oxygen; therefore, something has to be 

 done about providing space in the body 

 where food can be satisfactorily digested 

 and absorbed into a transporting medium 

 that will deliver it to each cell of the body. 

 The same transport system can be used that 

 carries the gases in respiration. All that is 

 needed is a place where food can be held 

 sufficiently long so that digestive enzymes 

 pouring into it will have time to break the 

 complex insoluble molecules down into 

 simpler soluble ones. Then proper facilities 

 will be needed for absorption, that is, large 

 surface areas, and so forth. 



These conditions are met in the digestive 

 tract of all higher animals very satisfac- 

 torily. They possess a portal of entry or 

 mouth, which may or may not be armed 

 with teeth for macerating food, and a long 

 tube into which digestive glands empty 

 their food-splitting enzymes. Undigested 

 food leaves through the end of the tube, 

 the anus. Once the food has reached the 

 soluble stas[e it is absorbed into the blood 

 and transported to each cell, which 

 picks and chooses the particular energy- 

 giving and constructive materials it needs 

 to perform its own specific job in the organ- 

 ism as a whole. 



Excretion. Closely linked with respira- 

 tion and nutrition is the matter of getting 

 rid of wastes, that is, excretion. In this dis- 

 cussion we shall consider only the nitrog- 

 enous wastes of metabolism as excretory 

 products, although in the broad sense of 

 the term excretion includes water and car- 

 bon dioxide. At the cellular level, nitrog- 

 enous wastes are also eliminated through 

 the plasma membrane. At the multicellular 



level, excretory products, if not removed, 

 accumulate very rapidly and soon reach a 

 point where the cell cannot survive be- 

 cause such products in quantity are toxic. 

 Hence, an effective excretory system is de- 

 manded in any animal where the cells are 

 removed any distance from the surface. 



Again the simple metazoan gets rid of its 

 nitrogenous wastes just as amoeba does, by 

 simple diffusion. Where there are but two 

 layers of cells, each in contact with the ex- 

 ternal world, the problem of excretion is 

 easily solved. 



In the complex metazoan, nitrogenous 

 wastes such as urea are removed by the 

 kidney, an organ designed so that all of 

 the transporting medium must pass through 

 it at regular intervals. By a process involv- 

 ing filtration, selective reabsorption, and 

 secretion (p. 524), wastes are removed 

 from the blood and conveyed out of the 

 body through a system of appropriate 

 tubes. All metazoan animals above the 

 simple two-layered animals possess such a 

 system of excretory tubules. In the lower 

 forms there are many units scattered among 

 the cells so that fluid bathing these cells can 

 find its way to one of these tubules and be 

 relieved of its load of nitrogenous wastes. 

 In higher forms the many excretory units 

 become compactly arranged in a single 

 organ, the kidney. 



Reproduction. Amoeba reproduces itself 

 by simply splitting into two offspring, the 

 most primitive type (fission) of reproduc- 

 tion found in living things (Fig. 4-13). 

 Whenever the amoeba reaches a certain 

 size, it divides and continues growing. The 

 rate at which it can increase its numbers 

 when conditions are favorable appears to 

 depend solely on the amount of food it can 

 engulf and the rate at which it can build 

 protoplasm. The daughter cells thus pro- 

 duced are all essentially alike and, barring 

 accidental death, live forever. 



Multicellular animals have nearly all 

 given up simple fission as a means of in- 

 creasing their numbers and have assigned 



