EXISTING ORGANISMS— PHYSIOLOGY 93 



fact are both. The genus Euglena includes a number of species 

 not unUke Sphaerella to the extent that they are green with chloro- 

 phyll and move about by means of a slender flagellum. It has 

 been found that cultures of Euglena kept away from light lose 

 their green color, and after a few generations become more defi- 

 nitely animals in appearance through the lack of chlorophyll. Such 

 colorless Euglenae can be kept alive by the addition of soluble 

 organic foods to the water in which they live, and since they are 

 kept away from sunlight, they carry on, obviously, the metabolism 

 of animals. Their production of chlorophyll is, however, merely 

 interrupted, for they again become green if brought into the light, 

 and carry on the metabolism of plants. They are definite connect- 

 ing links between the otherwise different kingdoms. 



Other Tjrpes of Metabolism. Although the method by which 

 both animals and plants liberate energy is the one most widely 

 prevalent in the organic world, it is not the only possil)le means. 

 Some of the bacteria carry on a fundamentally different process of 

 metabolism in which energy is secured by the oxidation of very 

 different substances. Beggiotoa and Thiothrix, the sulphur bac- 

 teria, for example, oxidize hydrogen sulphide and store up sulphur 

 as a by-product; this is later oxidized and excreted as sulphuric 

 acid. At least one of the iron bacteria, Spirophyllum ferrugineum, 

 is equally dependent upon ferrous carbonate as a source of energy. 

 Still other plants, the yeasts and anaerobic bacteria, do not need 

 any free oxygen to liberate energy, but accomplish this result by 

 modification of food substances in a way illustrated by ordinary 

 fermentation. In this process yeasts break down sugars, forming 

 alcohol and carbon dioxide and releasing the energy which the 

 organisms require. 



Specialization. It is of little or no use to consider the similarity 

 of details of physiological processes, for it is difficult to compare 

 the functions of different organs in animals which are not closely 

 related, and in similar organs similarity of function would be a 

 natural consequence. However, some generalizations are possible 

 in connection with the fundamental physiological properties of 

 matter and their distribution in the complex organism. The 

 ectoderm of Metazoa, since it retains direct contact with the 

 environment, might be expected to display a greater development 

 of the quality of irritability and this is true even in the highest 

 phylum, where it is the source of the highly developed nervous 



