PHYSIOLOGY OF THE MICROBES 99 



studying infusoria and amoebae rather than sharks and 

 elephants. 



Life means always the combustion of protoplasm, according 

 to Lavoisier's dictum, and this protoplasm replaces its losses 

 by assimilating food. Three methods of assimilation we know : 

 the chlorophyllous plants with the help of sunlight decompose 

 the carbonic acid of the air, manufacture hydrocarbons, and 

 finally turn the starch into more complex substances which go 

 to build up their protoplasm ; the green plant thus starts with 

 non-organised substances for all its food-supply ; animals feed 

 on plants or on other animals which have already fed on 

 plants : parasites absorb food which has already been prepared 

 ready-made for them by their hosts. All three methods 

 exist in the protozoa. 



There are some which, possessing chlorophyll, manufacture 

 their food exactly like green plants : these are the plant-animals 

 which form the link between the two kingdoms. They 

 prepare by synthesis a starch or a para-starch (Biitschli) and 

 can satisfy their life conditions in water containing mineral 

 salts, provided they receive the light of the sun. Certain 

 species lose their chromatophore granules when ready-made 

 food is supplied to them, permitting them thus to dispense 

 with the labour of " photo-synthesis." Certain flagellates live 

 in symbiosis with green algae which supply them with starch : 

 in such a case the protozoon may be said to be attacked by a 

 useful infection : further, this infection may be conveyed from 

 one to another. 



The other protozoa capture their prey, frequently in the form 

 of living creatures. Whether they have a mouth or not, whether 

 the food penetrates their bodies with the help of a current 

 produced in the water ^by cilia or flagella, or whether 

 it is the protoplasm of the protozoon itself which issues 

 from its envelope, introduces itself into the body of the 

 prey, and thus devours it from the inside, as it were re- 

 versing the roles, in all cases the important point in the 

 nutrition is intracellular digestion : the food is enclosed in a 

 little spherule inside the protoplasm where little by little it is 



H 2 



