ii NUTRITION 245 



alternation of an active or motile with a stationary or resting 

 condition. 



In the matter of nutrition, the differences between 

 Sphserella and Amoeba are very marked, and indeed funda- 

 mental. As we have seen, Sphaerella has no pseudopods, 

 and therefore 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 course 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. Never- 

 theless it must take food in some way or other, or the de- 

 composition 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 solution, espe- 

 cially 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 consider its protoplasm to be 

 constantly permeated by a very weak saline solution, the 

 most important elements contained 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. 



It water, containing a large quantity of Sphserella 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 has 



