168 MANUAL OF ELEMENTARY ZOOLOGY 



The nitrogen, sulphur, and phosphorus for this purpose are 

 obtained by most plants as salts in solution in the water 

 they take in, by their roots or sometimes by the whole 

 surface of the body. The green bodies of Hydra obtain 

 these elements in certain waste products of the metabolism 

 of the animal which they absorb. It may be that the Hydra 

 absorbs from them in return the excess of carbohydrates 

 which they form ; and this would account for the absence from 

 them of starch, which is so constantly found in plants. 

 Thus there is between the two organisms a partnership, in 

 which the animal benefits by the removal of waste products 

 and the supply of oxygen and possibly of carbohydrates, 

 and the plant benefits by the rich supply of nitrogenous 

 material and carbon dioxide. Such a partnership is known 

 as symbiosis and is in strong contrast with parasitism, in 

 which one of the partners benefits at the expense of the 

 other. Hydra, Zoochlorella, and Polytoma are examples of 

 the three kinds of nutrition found among living beings. 

 While the food of animals, which consists of complex organic 

 substances, is usually in the state of a solid or of the viscous 

 liquid of protoplasm, and has to be swallowed through an 

 opening, the materials taken in by green plants are simple 

 inorganic substances which can be absorbed as gases or 

 liquids through the surface of the body. Plants which have 

 no chlorophyll, such as fungi, and some animals which live 

 as parasites or in decaying matter, absorb their nourishment 

 through the surface of the body, but take it in the form of 

 organic substances, more or less complex in various cases, 

 derived from the bodies of other organisms. Thus there 

 can be distinguished three kinds of nutrition : (r) holozoic, 

 in which the food is organic and is swallowed ; (2) holophytic, 

 in which the food is inorganic and is absorbed through the 

 outer surface ; (3) saprophytic, in which the food is organic 

 and is absorbed. 1 It must, however, be understood clearly 

 that these differences concern only the form and manner in 

 which food enters the body. The food which is incor- 



1 The modes of nutrition classed under the general title " saprophytic " 

 vary greatly in detail, ranging from cases in which the food substances 

 are not much more complex than that of a plant, save that sugar is 

 substituted for carbon dioxide, to cases which differ from holozoic 

 nutrition only in the way in which the food enters the body. 



