No. 603] 



ECOLOGY OF THE PROTOZOA 



169 



insects (especially the HydrophUidcB), Limnea, Physa, 

 Planorhis, etc. Temperature of water, cir. 70° F, 

 Predominant forms : 



Motias fluida, very abundant 



Other forms present in less numbers (few or rare) : 



The species which I identified as Arcella discoides (Fig. 

 17) was much smaller than usual. Although there is con- 

 siderable variation in size among the members of Arcella 

 and particularly among Arcella discoides and vulgaris, 

 yet I have never seen recorded individuals so small. The 

 average diameter of twenty-five individuals from one 

 sample was 70 microns. 



The Front onia species (Figs. 18 and 19) are very fully 

 equipped with trichocysts. When irritated with acetic 

 acid and then stained with iodine or methyl green they 

 show beautifully. For use in the laboratory in the demon- 

 stration of the trichocysts of the ciliata they offer the best 

 possible material. The species are not very common, how- 

 ever, and I know of no culture medium. 



Trepomonas agilis (Fig. 20) varies greatly in size. Its 

 apparent variation in shape can be explained, I think, by 

 the fact that it swims sometimes with one aspect of the 

 body presented to the observer and sometimes with an- 

 other. The curious irregularity of its body would there- 

 fore allow it to show a number of different outlines. Often 

 it swims in one position for a long period, and again it 

 twirls rapidly about going through the whole gamut of its 

 apparent changes of form in a few seconds. 



In one typical roadside ditch overhung with grasses and 

 weeds and literally filled to overflowing with rich masses 

 of Spirogyra I found an almost pure culture of Euglena 



vniamyaomonas sp., numerous 

 Difflugia glohulosa, numerous 

 Euglena viridis, very abundant 



Actinophrys sol 

 Arcella discoides 



Arcella vulgaris 



Colpoda campyla 

 Cyphoderia ampulla 

 Frontonia sp. 



