ually and were subject to a process of sedimentation; but if such were the fact, the beds of our streams would present a very different appearance. It is also highly improbable that an actual settlement of such extremely small organisms can take place in a stream having even a moderate current. Obviously some of the bac teria will be dragged to the bottom along with the heavy matter in suspension, but the great. majority will not settle. Experiments in this direction have been made for improving the condition of the water in large aquaria and fish ponds by the precipitation of clay; but few have thought, of the natural biological processes of water purification, because an unexpectedly extensive view of these processes has been gained only recently by planktological and thorough hydrobio logical investigations.Other Food Consumers.-Among the organisms that are carried in suspension in water, the minute Crus tacea must also be considered. They are found in all seasons in the plankton of rivers, although in less quan tity in swiftly-flowing streams. At one time the Clado cera, or water fleas, will predominate, while at another time the Copepoda will be the more numerous, accord ing to their cycle of propagation. Their significance for the purification of wat.er has likewise not been sufficiently appreciated. Concerning the quality of their food, numerous but not conclusive observations have been made. Some investigators assert that they live only on animal food, such as protozoa, while others claim that they use only vegetable food, such as dia toms and other minute algoo; but it must be remembered that both protozoa and minute algoo usually adhere to decaying detritus of animal and vegetable origin.An enormous multiplication of Daphnia has been observed in highly-polluted village ponds, in which, however, various oxygen-producing algoo, and especially the euglena previously mentioned , rendered respiratIOn or breathing possible. The question is, what becomes of the great quantities of offal and excret.a, the many remnants of decaying plants, the refuse of communities, and the finely divided factory wastes of every de scription , which find their way into our streams, even under normal conditions, if a large portion thereof is not consumed by the aquatic detritus-eaters and the omnivorous fauna before settling to the bottom. By increasing the discharge of sewage into a river, a very large increase in the number of water-fleas has resulted , as was observed at Hamburg and elsewhere; and be cause the substantial value of the crustacea is materially greater than that of the rotat.oria, the production of living animal �ubstance is greater at all times in the sewage-polluted portion or the river than in the clean water portion. Most of the larger crustacea are omni vorous, and many of them do not spare even theIr own progeny.
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