PROTOZOA. 71 



digestive cavity is present, which constitutes one of the 

 important features of animal life ; and as we ascend in the 

 scale, the digestive cavity or stomach becomes more and 

 more specialized for the purpose of receiving and digesting 

 solid organic food. Now this requirement of organic food 

 obviously limits the range of animal life to those places 

 where such food exists. As a set-off against this apparent 

 drawback to the extension of animal life, it must be borne 

 •in mind that organic is much more easily assimilated than 

 inorganic matter, which forms the staple of plant food ; 

 inasmuch as the chemical energy required to convert 

 inorganic into organic food is already accomplished, and 

 consequently a surplus of energy is left to the animal, to be 

 expended in the development of other functions than those 

 required for preparing the indispensable food supply. As a 

 result of the above conditions — and more especially as 

 progressive differentiation necessitated special kinds of 

 organic food — the power of locomotion, universal in the 

 higher forms of animal life, became a necessity. 



Returning to the Protozoa, which as a group are con- 

 sidered to belong to the animal side of hfe, numerous forms 

 are met with differing so far as is known from typical 

 Protozoa only in the presence of chlorophyll, which as 

 already stated is one of the leading features of plant life; 

 and although chlorophyll is not considered at this stage as 

 absolutely stamping the plant nature of the body producing 

 it, yet it enables the organism to feed on inorganic food, 

 and as this tendency becomes more pronounced, so in pro- 

 portion the power of feeding on organic food degenerates 

 until eventually it is completely lost, and the organism is 

 then considered as a plant belonging to the group Schizo- 

 phycecB, still aquatic in habit, and frequently possessed of 



