EVOLUTION OF PROTOZOA iii 



Evolution of Single-Celled Animals or Protozoa 



We have no idea when the first unicellular animals known 

 as Protozoa appeared. Since the Protozoa feed freely upon 

 bacteria, it is possible they may have evolved during the bac- 

 terial epoch; it is known that Protozoa are at present one of 

 the limiting factors of bacterial activity in the soil, and it is 

 even claimed^ that they have a material effect on the fertility 

 of the soil through the consumption of nitrifying bacteria. 



On the other hand, it may be that the Protozoa appeared 

 during the algal epoch or subsequent to the chlorophyllic plant 

 organisms which now form the primary food supply of the 

 freely floating and swimming protozoan types. A great num- 

 ber of primitive flagellates are saprophytic, using only dis- 

 solved proteids as food.- 



Apart from the parasitic mode of deriving their energy, 

 even the lowest forms of animal life are distinguished both in 

 the embryonic and adult stages by their locomotive powers. 

 Heliotropic or sun reactions, or movements toward sunlight, 

 are manifested at an early stage of animal evolution. In this 

 function there appear to be no boundaries between animals 

 and the motile spores, gametes, and seedlings of certain plants.^ 

 As cited by Loeb and Wasteneys, Paul Bert in 1869 discovered 

 that the little water-flea Daphnia swims toward the light in all 

 parts of the visible spectrum, but most rapidly in the yellow or 

 in the green. More definitely, Loeb observes that there are 

 two particular regions of the spectrum, the rays of which are 

 especially effective in causing organisms to turn, or to congre- 

 gate, toward them; these regions lie (i) in the blue, in the 



'Russell, Edward John, and Hutchinson, Henry Brougham, 1909, p. 118; 1913, pp. 

 191, 219. 



2 Gary N. Calkins. 



' Loeb, Jacques, and Wasteneys, Hardolph, 1915.1, pp. 44-47; 1915.2, pp. 32S-330. 



