166 THE BIOLOGY OF AN ANIMAL. 



free by this action the energy which maintains their vital ac- 

 tivity. And, sooner or later, both give back to the environment 

 the matter and energy which they have taken from it. In 

 other words, both effect an exchange of matter and of energy 

 with the environment. 



Nevertheless the plant and the animal do differ. They differ 

 widely in form, and the plant is fixed and relatively rigid while 

 the animal is flexible and mobile. The body of the plant is 

 solid ; that of the animal contains numerous cavities. The plant 

 absorbs food directly through the external surface; the animal 

 partly through the external and partly through an internal 

 (alimentary) surface. The plant absorbs simple chemical com- 

 pounds from the air and earth, and kinetic energy from sunlight ; 

 the animal absorbs, for the most part, complex chemical com- 

 pounds and makes no nutritive use of the sun's kinetic energy. 

 By virtue of this energy the plant is able to manufacture starch 

 from simple compounds, carbon dioxide and water; the animal 

 lacks this power. The plant can build up proteids from the 

 starch and other compounds of its food; the animal absolutely 

 requires proteids in its food. And by manufacturing proteids 

 within its living substance, the plant is relieved of the necessity 

 of carrying on a process of external digestion in order to render 

 them diffusible for entrance into the body. 



And yet, great as these differences appear to be at first sight, 

 all of them, with a single exception, fade away upon closer exam- 

 ination. This exception is the mode of nutrition. Plants and 

 animals differ in form because their mode of life differs; but a 

 wider study of biology reveals the existence of innumerable ani- 

 mals which have a close superficial resemblance to plants (corals, 

 sponges, hydroids, etc.), and of many plants which resemble 

 animals, not only in form, but also in possessing the power of 

 active locomotion. The plant has no use for a stomach because it 

 is bathed over its whole surface by food-materials which make 

 their entrance freely by simple diffusion. Moreover, the stomach 

 of the worm, as shown by its development, is really a part of the 

 general outer surface which is folded into the body; and the 

 animal, like the plant, therefore, absorbs nutriment over its whole 

 surface oxygen through the general outer surface, other food- 

 matters through the infolded alimentary surface. 



And in like manner it is easy to show that not one of the dif- 



