STATE GEOLOGIST. 7 



The fact is, that the principle laid down by the Deacon (of 

 venerable memory) that ''the weakest pint must stand the strain/^ 

 maintains in nature aside from the nature of "shays." The minutest 

 forms are in some sense the most important, for they are the links 

 wliich stand nearest the rock, and it they be loosened the depen- 

 dent series falls. 



The animals of the above group are, it is likely, the best criteria 

 by which to judge of the purity of natural waters if their distribu- 

 tion were correctly understood. The presence of some species in 

 great numbers is sufficient evidence of organic impurity. A criti- 

 cal study of the contents of samples of such waters will enable us 

 to determine their character almost as well as by analysis. The 

 following list of the animal life visible in a quart of filthy pond- 

 water, taken by simple dipping, will perhaps be suggestive on this 

 point: 



Daphnia pulex 6 



Cerioclaphnia 1400 



Simoccplialus 56 



Cypris.. 50 



Cyclops 30 



Sand-fleas 120 



Total Crustacea 1662 



Infusoria 35 



Arachnida {Hydrachna) 1 



Vermes . 5 



Insecta— 



Coleoptera (larvae) 8 



Diptera (larvae) 11 



Hemiptera 10 



Mollusca 35 



Total n 1767 



The above are simply the animal forms visible to the (trained) 

 unassisted eye; the truly microscopic forms number vastly more. 



But each gathering includes specimens of carnivorous entomost- 

 raca as well, and these are not less interesting and bizarre than the 

 cladocera. 



The common cyclops, busy picking the bones of a luckless pol- 

 liwog (must we say pnrwiggy?), is not less benevolent than the 

 animate filters mentioned above. The amount of such material 

 that they will dispose of in a short period of time is truly astonish- 

 ing. It is the province of the following chapters to describe 

 briefly such of these animals as fall in the two groups Cladocera 

 and Copepoda and have been noticed in America. 



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