RESPIRATION IN AMCEBA 17 



It is not easy, if it be possible at all, actually to observe the 

 process of respiration in so small an animal as an Amceba, but 

 we know perfectly well from the analogy of higher organisms 

 what must take place. Oxygen is required for the combustion of 

 the protoplasm from which the energy of the organism is 

 derived, and this oxygen occurs in a state of solution in all 

 ordinary water which is exposed to the air. At the same time 

 carbon dioxide, or carbonic acid gas, must be produced as one of 

 the products of the combustion, by oxidation of the carbon in the 

 protoplasm. This waste product (C0 2 ) will first of all be 

 dissolved in the water which forms the greater part of the bulk 

 of the living organism, while at the same time the water by 

 which the animal is surrounded may be regarded as a very 

 dilute solution of oxygen. The outermost layer of the ectoplasm 

 may be looked upon as a thin membrane separating the two 

 solutions. 



We know from experiment that whenever two gases, or solutions 

 of gases, of different density, are separated from each other by a 

 thin organic membrane, they will pass through that membrane 

 in opposite directions until a state of equilibrium is established 

 between the two. This process of osmosis or diffusion is, as we 

 have already seen, the essential feature of respiration in all plants 

 and animals, although probably the purely physical process is 

 controlled to some extent by the living protoplasm. 



In the case of the Amoaba then, the carbon dioxide diffuses 

 out through the ectoplasm into the surrounding water while 

 the oxygen from the surrounding water diffuses in, and the 

 necessary exchange of gases is brought about. No specialized 

 organs of respiration, such as we meet with in the higher 

 animals, are required. The whole surface is a respiratory sur- 

 face, and all parts of the interior are within reach by the simple 

 process of diffusion, aided doubtless by the circulation of the semi- 

 liquid protoplasm which is constantly going on inside the body. 



Other waste products must be formed by the combustion of the 

 protoplasm in addition to carbon dioxide. What these are we 

 do not exactly know in the case of the Amoeba, but it is evident 

 that they must contain nitrogen, which is one of the' essential 

 constituents of all proteids. These waste products must be got 

 rid of by some process of excretion, and it is usually supposed 

 that they are passed in a state of solution to the contractile 

 vacuole and thence periodically expelled to the exterior, so that 



B. c 



