STRUCTURAL CHARACTERS OF ANIMALS 393 



between animals on the one hand and green plants on 

 the other, vary very much in respect of the form in 

 which they can assimilate nitrogen. Most bacteria 

 take it in complex compounds, but some in simpler ones, 

 while others can fix free nitrogen. The fungi proper 

 show a similar but more restricted variability. The 

 green plants, on the whole, take their nitrogen in the 

 form of simple salts, such as nitrates, but even they 

 show variability. Some of the lowest forms can take 

 their nitrogen as complex organic compounds, and even 

 some of the seed plants (which as a class only use 

 nitrates) can feed on organic nitrogenous compounds. 

 This variability doubtless arose very early in the history 

 of life, if it did not exist from the very beginning, and 

 was the origin of the great differences of structure and 

 mode of life in existing organisms. 



The animals are the most specialised class. Organisms 

 which can only take nitrogen in the form of proteins, 

 and thus from organic sources, were forced, so to speak, 

 into certain structures and habits, or they could not 

 survive. They must either have a naked protoplasmic 

 surface, as amoeba has, through which solid food can 

 be directly ingested, or alternatively they must have a 

 special opening in the surface (mouth) through which 

 food can be taken in. A further development, found 

 in the great majority of animals is the existence of 

 a cavity in the body (gut) into which the mouth leads 

 and where the food is digested — broken up and rendered 

 soluble — so that it can be absorbed by the living proto- 

 plasm ; and an exit from this cavity (anus) through 

 which the indigestible residue (fences) can be discharged. 

 A still further development is a system of tubes 

 (circulatory system, vascular system) which carries the 

 soluble products of digestion to the living cells of 



