MULTICELLULAR ANIMALS 153 



devices that serve the animal as mechanisms for the functions of 

 nutrition, of irritabiUty, of reproduction, of contractihty, of excre- 

 tion, and their various associated structures. The detail of the many 

 types of organ systems is so great as to prohibit any extended treat- 

 m.ent here; only the major facts will be discussed. 



First and perhaps most important in the life of all organisms is 

 the matter of nutrition, for on nutrition rests the whole complex o£ 

 life; both energy and materials are wholly derived from the func- 

 tioning of the mechanisms of nutrition. 



Nutritive Mechanisms. Methods of obtaining nutriment are 

 fundamentally different in plants and animals and may be regarded 

 as largely responsible for their very different structure and life 

 habits. In plants, particularly the green plants, the immediate source 

 of energy is the energy of sunlight; materials for their subsistence 

 are in a crude state as compared with the materials necessary for 

 animal nutrition. Thus the green plant has only to be located in a 

 soil containing relatively simple substances and to have available a 

 supply of carbon dioxide and the presence of sunlight. None of 

 these require extreme sensitivity to the environment, nor rapid 

 motility, nor means of grasping and breaking up materials; hence 

 plants are relatively non-motile and are able to thrive for long 

 periods in the same location. On the other hand, the principles of 

 holozooic nutrition impose on animals the necessity of feeding 

 either on plants or of feeding on animals that have fed on plants. 

 They require powers of locomotion, of sensing the presence of 

 food, of grasping a nutritive material and of breaking it up, and of 

 rejecting from the body those substances that are not available for 

 assimilation into animal protoplasm. 



Fundamentals of an Animal Digestive System. Methods 

 of securing food vary widely among animals; some involve the 

 development of complex traps for capturing edible forms, for ex- 

 ample, spiders' webs; some require great muscular power and speed 

 in effecting captures, for example, wolves,, cats, or other Carnivora; 



