€H. X] THE CONDITIONS OF LIFE IN THE SEA 223 



tion are essentially similar to those which are carried on in the 

 warm-blooded animal. The " raw materials of the food-stufifs " 

 iire (1) the jDroteids of the food organism. These are digested 

 in the alimentary canal, and the soluble peptones so formed are 

 -absorbed by the intestinal walls, and, in this process, are converted 

 into the proteid which is characteristic of the animal concerned. 

 (2) The carbohydrates, which are hydrolysed during digestion so 

 as to form diffusible sugars w^hich are then absorbed by the intes- 

 tinal walls. (3) The fats, which are dissociated in digestion and 

 are again synthesised in the process of absorption. The undigested, 

 or undigestible residue of the food is expelled from the alimentary 

 canal as the faeces. The products of digestion, that is, the re- 

 converted specific proteids, the sugars, and the reconverted fats are 

 carried in the blood stream to the tissues, there to be built up 

 into living substance, or to form stores of reserve materials ; and 

 ultimately to undergo combustion by the inspired oxygen, yielding 

 in this process the energy of the animal. Some of this expended 

 tissue substance is excreted in a completely oxidised form as 

 ■carbon dioxide and water, and the remainder as the incompletely 

 oxidised products, urea, uric acid, guanin or hippuric acid. So 

 far we are on familiar ground, and processes, characteristic of the 

 metabolism of the warm-blooded animal, are carried on also in 

 the life economy of the higher marine animals. 



Holophytic organisms. In complete contrast with these 

 processes stand those which are generally characteristic of the 

 higher plants. Here the " raw materials of the food-stuffs " are 

 the mineral salts, ammonium sulphate and carbonate, alkaline 

 nitrates and the carbonic acid of the sea water. The typical 

 plant builds up starch from the simple compounds carbonic di- 

 oxide and water, and amido-substances from the mineral salts 

 taken up by its roots. From these substances it prepares the 

 proteid of its tissues. This extraordinary power of forming starch 

 is called jDhoto-synthesis, and it depends on the existence, in the 

 green parts of the plant, of the pigment, chlorophyll. This 

 substance can intercept the energy of sunlight and utilise it in 

 the elaboration of starch. 



Carbon dioxide and water, the two compounds from which the 

 starch is built up, are completely oxidised substances. But starch 



