CHAPTER X 

 THE ANIMAL BODY 



If we contemplate the method of Nature, we see that every- 

 where vast results are brought about by accumulating minute 

 actions. Spencer. 



THE most obvious characteristic which distinguishes fa- 

 miliar plants and animals is the power of locomotion of the 

 latter. This criterion, however, fails among the lowest forms; 

 for example, Sphaerella, as we have seen, swims as actively as 

 Paramecium. Moreover, among multicellular animals there 

 are innumerable sessile forms, such as the typical stages of 

 the Sponges, Hydroids, Barnacles, etc. Although the power 

 of locomotion is not a diagnostic character of animals as com- 

 pared with plants (this, as has been explained, being chiefly a 

 matter of metabolism), it is a fact that, taken by and large, 

 the great dissimilarity between the bodies of multicellular 

 plants and animals is a direct or indirect result of the loss by 

 plants and the development by animals of the primitive power 

 of locomotion which most unicellular organisms possess. At 

 the basis of this difference is probably the fact that early in 

 the evolution of plants comparatively rigid cell walls of 

 cellulose were established, which directed the development of 

 the body along relatively fixed lines. On the other hand, 

 animal cells, unhampered by the limitations imposed by rigid 

 confining walls, were free to respond in more ways to environ- 

 mental conditions, and this made possible the extremely 

 diverse forms of mobile bodies characteristic of the animal 

 kingdom. This greater plasticity of the animal in compari- 



115 



