INTRODUCTION. 7 



animals among the protozoa, such as the amoeba, have no stomach, the 

 general surface serving not only for the purpose of digestion, but also 

 for absorption, an extemporaneous stomach being formed by wrapping a 

 part of the external general body surface around the substance to be 

 digested. 



So also in the tape-worms and other parasitic forms of animal life, 

 there is an entire absence of any special aperture for the entrance of 

 nutritive matter, such organisms living by the simple imbibition of 

 nutritive matter in solution. 



When, however, we examine into the nature and mode of assimila- 

 tion of food, the nutritive processes occurring in the interior of the 

 organism, and the results of the conversion and assimilation of food, 

 then only have we any reliable scientific data for distinguishing animals 

 from plants. In the first place, the food of animals differs from that of 

 plants in its nature. Animals require organic food ; plants live on inor- 

 ganic or mineral matter. The nutritive processes in the two kingdoms 

 are also diametrically opposed : the plant absorbs water, ammonia, carbon 

 dioxide and certain salts, and out of these manufactures the albuminoids, 

 carbohydrates and hydrocarbons found in vegetable tissue. The 

 animal feeds on these complex vegetable compounds, — and this holds 

 whether the animal be herbivorous or carnivorous, — and returns to the 

 soil and atmosphere the inorganic matter from which they were manu- 

 factured by the plant; and in the same form, i.e., carbon dioxide, water, 

 ammonia, and certain salts. The plant therefore converts simple inor- 

 ganic compounds into complex organic compounds, while the animal 

 reduces complex organic matter to its simple inorganic constituents. 



A further point of distinction between animals and vegetables, and 

 one closely connected with the nutritive processes, is their behavior to 

 the atmosphere. The animal recpiiires for the processes of reduction 

 already mentioned as constituting its mode of nutrition a constant supply 

 of oxygen, which is withdrawn from the atmosphere and returned to it 

 in the form of C0 2 , representing one of the end products of oxidation 

 of the carbon of its tissues and food. Plants, on the other hand, absorb 

 C0 2 , and under the influence of sunlight, by the action of their chloro- 

 phyll, break up this C0 2 . fix the carbon in their tissues, and set free 

 oxygen into the air. The plant thus absorbs what the animal excretes, 

 and the animal absorbs what the plant excretes. We thus see that 

 animals and plants offer striking points of contrast as to the character 

 of their food and the nature of their nutritive processes, and, although 

 there are several apparent exceptions to the general outline here given, 

 their consideration may be deferred to the chapters on the Chemical 

 Processes in Cells. 



We have found now that all objects in nature must be either organic 



