80 



THE ORGANIZED ANIMAL 



of many parts very much alike. Once this 

 happened, an integrating system could be 

 devised which would then result in a seg- 

 mented animal such as an earthworm. This 

 may be the way segmentation came about. 

 In any case, segmentation is a persistent 

 character of many higher animals, includ- 

 ing man. Lower forms such as the earth- 

 worm show both internal and external seg- 

 mentation, and most of its organs are 

 duplicated in almost all of the segments. 

 However, some organs, like the gonads, are 

 confined to certain segments, and, in gen- 

 eral, there is a concentration of nervous 

 organs at the head end of the animal. 



Segmentation is not very obvious in such 

 higher animals as man because it is ob- 

 scured by the specialization of individual 

 segments. Nevertheless, a glance at the 

 skeleton shows that the basic plan is seg- 

 mental (Fig. 15-2). The vertebrae and the 

 ribs, while varying slightly in different parts 

 of the body, are serially repeated and re- 

 semble one another very closely. Segmenta- 

 tion is clearly indicated in the early em- 

 bryos of vertebrates, and during the first 

 few weeks of development the human em- 

 bryo shows segmentation which in principle 

 is like that of the earthworm. As it grows 

 older the clear-cut segments are obliter- 

 ated by fusion and reorganization. 



With the gradual development of these 

 various features, animals became organized 

 into what they are today. Many new prob- 

 lems arose with the ever increasing com- 

 plexity of the total organism, and the con- 

 sequences of these we shall consider a little 

 further. 



THE CONSEQUENCES OF 

 ORGANIZATION 



As animals grew in bulk and complexity 

 they were confronted more and more with 

 the problems of transport and coordination. 

 Such activities as respiration, nutrition, and 

 excretion, which were simply performed 

 when the cell was in constant contact with 



its fluid world, became difficult or im- 

 possible when it was separated from this 

 environment by even a few covering layers 

 of cells. Such inner cells would have to de- 

 pend on diffusion to carry oxygen and food 

 to them and to remove wastes from them. 

 At best this is a slow process and certainly 

 not rapid enough to allow an animal to 

 grow very big or become very active. 

 Therefore, specific organ systems had to 

 be evolved if animals were to grow in bulk 

 and activity. 



In the following discussion we shall com- 

 pare the activities of organisms at the cellu- 

 lar and multicellular levels, pointing out the 

 problems involved in becoming complex 

 and indicating how the metazoan animal 

 has solved them. We can use amoeba for 

 the single-cell level, hydra for a simple 

 metazoan, and man for the multicellular 

 level. 



Respiration. Respiration is the taking in 

 of oxygen and the eliminating of carbon 

 dioxide. In the amoeba this is cared for very 

 efficiently and simply by diffusion through 

 the limiting membrane that envelops the 

 cell (Fig. 4-12). Once in the cell, oxygen 

 diffuses to where it is needed and the car- 

 bon dioxide which results from the combi- 

 nation of oxygen with food likewise makes 

 its way to the cell or plasma membrane, 

 passing out through it to the surrounding 

 water. The only essential need is sufficient 

 oxygen in the environment. 



Respiration is essentially the same in a 

 simple metazoan such as hydra where each 

 cell is in contact with its external world. 

 Diffusion is adequate to take care of the 

 respiratory needs of this simple animal. 



In a complex metazoan animal, respira- 

 tion is, of course, the same, and oxygen and 

 carbon dioxide exchange must take place in 

 each cell. Since most of the cells lie deep 

 within the organism there is no possible 

 chance for gaseous exchange by diffusion 

 with the external world, especially since 

 the organism is covered with skin which is 

 impervious to such gases. The metazoan 



