68 



THE ORGANIZED ANIMAL 



strength in union. Secondly, division of 

 labor was initiated among the cells, some 

 becoming sterile and functioning only in 

 locomotion and food-getting, whereas 

 others retained the primitive condition of 

 colony-reproducing. Some of the reproduc- 

 tive cells became highly modified into eggs 

 and sperms while others merely retained 

 their primitive characteristics of reproduc- 

 ing by simple fission. In other words, a dif- 

 ferentiation of function took place among 

 the cells of the aggregate, definitely mark- 

 ing it off from the isolated single cells and 

 at the same time creating the first step in 

 the organization of a complex animal 

 through the loss of the power of reproduc- 

 tion by some of the cells. Once this step 

 was taken, differentiation of the soma 

 cells continued in various directions toward 

 greater and greater complexity, and thus 

 up the long trail to such highly intricate 

 forms as man. 



This gradual advance in complexity 

 might be compared to the evolution of our 

 own society. The Protozoa may be com- 

 pared to primitive man who lived alone and 

 was compelled to obtain all of his own food, 

 make his own clothing, and provide his own 

 shelter. Existence by this crude means 

 made chances for survival poor, and mor- 

 tality high. Later, man associated himself 

 with others in the common interest of sur- 

 vival and of making the drudgery of life 

 less grueling. The first groups were made 

 up of the immediate family; they lived to- 

 gether, hunted together, and made shelters 

 together. In other words, they performed 

 all the duties acting as a group rather than 

 singly as heretofore. Food was easier to 

 secure because they could surround and kill 

 larger animals, their shelters could be more 

 elaborate, and the burdens which fell upon 

 each individual were not as great as when 

 each lived alone. Such aggregation was 

 continued to include larger groups until 

 small villages were formed; with the in- 

 creasing numbers of individuals partici- 

 pating in mass efforts, less responsibility fell 



to each one, and what was more important, 

 each shared in the results of the mass ef- 

 forts. They aU lived better and longer. This 

 has continued and finally developed into 

 our modern civilization. There are places 

 on the earth today where primitive peoples 

 live just as they did many thousands of 

 years ago. These people are unsuccessful, 

 biologically speaking, because they have 

 been unable to spread their kind over the 

 world. Such is the criterion of biological 

 success. Following this analogy, we can 

 think of primitive society as resembling the 

 single-celled animal and modern society as 

 the complex metazoan, such as the mam- 

 mal. As tlie cells began to aggregate into 

 groups, individual cells specialized in par- 

 ticular jobs, and the group as a whole be- 

 came more complex. There are animals all 

 along the evolutionary scale which repre- 

 sent steps in increasing complexity. 



The next step toward a more complex 

 animal is a simple metazoan with further 

 differentiation among its soma cells. The 

 body is now a simple sac composed of two 

 layers of cells. Hydra is an excellent ex- 

 ample, and while more will be learned 

 about it in a later chapter, we may briefly 

 examine its anatomy at this point in order 

 to carry further the idea of increasing com- 

 plexity. 



Hydra is made up of many cells, mostly 

 soma cells, which are arranged in two lay- 

 ers (Fig. 4-1). Some of the cells in the out- 

 side layer (ectoderm) have differentiated 

 into "nettle cells" for stinging purposes in 

 defense or offense. Others are able to 

 lengthen and shorten during locomotion 

 and to convey impulses (neuromuscular 

 cells). Still others have the ability to give 

 rise to sperms and eggs, and to new indi- 

 viduals by bud formation. Here, then, we 

 see that the soma cells have differentiated 

 into several kinds, while the sex cells re- 

 main much like they were in Volvox. Divi- 

 sion of labor has started among the soma 

 cells which is the next step in the develop- 

 ment of more complex animals. 



