Less, xvii DIFFERENCES IN NUTRITION 177 



only by a delicate cuticle interrupted at the mouth and anus, 

 while in Mucor and Vaucheria the outer layer is formed by 

 a firm, continuous covering of cellulose. 



We thus have as the first morphological difference between 

 our selected animal and vegetable organisms the absence of 

 a cellulose cell-wall in the former and its presence in the 

 latter. This is a fundamental distinction, and applies 

 equally well to the higher forms. The constituent cells of 

 plants are in nearly all cases covered with a cellulose coat 

 (p. 60), while there is no case among the higher animals of 

 cells being so invested. 



Next, let us take a physiological character. In all three 

 organisms there is constant waste of substance which has to 

 be made good by the conversion of food material into proto- 

 plasm : in other words, constructive and destructive meta- 

 bolism are continually being carried on. But when we come 

 to the nature of the food and the mode of its reception, we 

 meet at once with a very fundamental difference. In Para- 

 moecium the food consists of living organisms taken whole 

 into the interior of the body, and the digestion of this solid 

 proteinaceous food is the necessary prelude to constructive 

 metabolism. In Vaucheria the food consists of a watery 

 solution of carbon dioxide and mineral salts — i.e., it is liquid 

 and inorganic, its nitrogen being in the form of nitrates or 

 of simple ammonia compounds. Mucor, like Paramoecium, 

 contains no chlorophyll, and is therefore unable to use 

 carbon dioxide as a food : like Vaucheria, it is prevented 

 by its continuous cellulose investment from ingesting solid 

 food, and is dependent upon an aqueous solution. It takes 

 its carbon in the form of sugar or some such compound, 

 while it can make use of nitrogen either in the simple form 

 of a nitrate or an ammonia salt, or in the complex form of 

 proteids or peptones. 



N 



