4 General Part. 



external stimulus, this simply gives the organism an occasion for 

 moving, The power of reacting to stimuli is called irritability. 

 The Amoeba is further characterised by building up its own substance 

 from materials which it has taken from the environment, i.e., it 

 feeds: it does this by surrounding other little organisms, and 

 inanimate particles, with its pseudopodia, and absorbing them into 

 its protoplasm, expelling, after a time, that part of the food which 

 cannot be assimilated. Besides these solid particles, the Amoeba 

 absorbs water, and also free oxygen, which is present in all natural 

 waters, and which is absolutely necessary for its existence, since it 

 cannot live in water which does not contain this gas, even if all other 

 necessary conditions are fulfilled. The oxygen unites with some of 

 the carbon present in the protoplasm, forming carbon dioxide, which 

 is got rid of ; the Amoeba is, as it were, a little machine, in which, 

 ]'ust as in a steam-engine, carbon is burnt ; a certain amount of 

 energy is liberated by this combustion, and is manifest as motion. So 

 m.uch is clear and certain, but there are many unknowi^ steps in the 

 activity of the little machine : by the combination of the oxygen and 

 carbon, the complex constituents of protoplasm are broken down, and 

 simultaneously, new compounds, principally water and nitrogenous 

 matter (uric acid), are formed. -The latter is of no use to the Amoeba, 

 but must be got rid of; it collects, as waste material dissolved in 

 water, in little cavities in the protoplasm, called vacuoles, and by 

 their contraction (or rather, the contraction of the protoplasm round 

 them) the contents are expelled. It is evident, therefore, that a 

 partial destruction of the complicated materials of the Amoeba is 

 constantly going on, so that its mass is diminished, but the food 

 taken in makes good this loss, and even produces a surplus, so 

 that the Amoeba grows — it actually increases in bulk. In close 

 relation with this is the last important quality of the Amoeba, its 

 power of multiplying by fission. In this process the nucleus 

 first breaks in two, a constriction of the protoplasm follows, and 

 finally, it is separated into two nearly equal pieces, each with its own 

 nucleus. 



All the characters just mentioned distinguish the Amoeba as 

 living, as an organism (though of the simplest kind), in contrast 

 to the lifeless particles which occur near it in the water. Death, 

 which is caused by a change in the external conditions {e.g. by too 

 great heat), deprives the Amoeba of all these qualities. 



In their principal features the rest of the Protozoa are 

 all essentially similar to the Amoeba. A few variations may 

 arise by the formation of a hard, protecting part (skeleton) of 

 lime or silica, in, or round, the protoplasm : or its outer layer 

 may be of a firmer consistency than the rest, so that, although 

 it retains its mobility (the movements of the granules and rough 

 alterations of shape, affecting the whole body, may still be observed) , 



