THE BIOLOGICAL UNIT 37 



the best illustrations of a free living biological 

 unit. The body is irregular in outHne with various 

 branches extending from the central mass. Even 

 a superficial glance reveals a nucleus, surrounded 

 by a somewhat homogeneous appearing area 

 which is the cytoplasm. 



There is nothing permanent or fixed about the 

 shape of this living cell. The animal moves about 

 by using some of its cytoplasmic extensions (Pseu- 

 dopodia) as legs to walk on, and some of them as 

 feelers to search for food. There is no mouth 

 nor stomach. The cell does not have to turn 

 around when a morsel of food is found. It simply 

 forces the food particles into the cytoplasm, and 

 moves on in search of more food. We say that 

 the food thus taken up is now digested, but no one 

 claims that he can actually prove this statement by 

 studying the ameba alone. One must reason from 

 analogy. The facts set forth in Chapter II page 

 15, indicate that chemical processes are similar 

 in all biological units. One can experimentally 

 prove that digestion is necessary in all higher 

 animals and the chemical changes are comparative- 

 ly well understood. There is no evidence that any 

 plant or animal is able to utilize food without 

 changing it so that the living protoplasm can use 

 it. From these reasons, very sketchily presented, 

 we judge that the ameba must digest the engulfed 

 particles of food. 



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