n NUTRITION 245 



alternation of an active or motile with a stationary or 

 resting condition. 



In the matter of nutrition, the differences between 

 Sphaerella and Amoeba are very marked, and indeed 

 fundamental. As we have seen, Sphaerella has no 

 pseudopods, and theiefore cannot take in solid food after 

 the manner of Amoeba ; moreover, even in its active 

 condition it is usually surrounded by a cell-wall, which 

 of cour^ quite precludes the possibility of ingestion. 

 As a matter of observation, also, however long it is 

 watched it is never seen to feed in the ordinary sense 

 of the word. Nevertheless it must take food in some 

 way or other, or the decomposition of its protoplasm 

 would soon bring it to an end. 



Sphaerella lives in rain-water. This is never pure 

 water, but always contains certain mineral salts in solu- 

 tion, especially nitrates, ammonia salts, and often 

 sodium chloride or common table-salt. These salts can 

 and do diffuse into the water which is a constituent part 

 of the protoplasm of the organism, so that we may con- 

 sider its protoplasm to be constantly permeated by a very 

 weak saline solution, the most important elements con- 

 tained in which are oxygen, hydrogen, nitrogen, 

 potassium, sodium, calcium, sulphur, and phosphorus. 

 It must be remarked, however, that the diffusion of 

 these salts does not take place in the same uniform 

 manner as it would through parchment or other dead 

 membrane. The living protoplasm has the power of 

 determining the extent to which each constituent of the 

 solution shall be absorbed. 



If water containing a large quantity of Sphaerella is 

 exposed to sunlight, minute bubbles appear in it, and 

 these bubbles, if collected and properly tested, are found 

 to consist largely of oxygen. Accurate chemical analysis 



