990 SUMMAKY OF CUEEENT EESEAKCHES EELATING TO 



whilst, on the contrary, the number of worm-like larv^ were more 

 than one per gallon. 



Now if the minute free-swimming animals thus increase when a 

 certain amount of sewage supplies them with ample food, it is quite 

 obvious that they must have a most important influence in removing 

 objectionable impurities. The number of escrements of entomostraca 

 in the recent mud of such rivers as the Thames is most surprising. 

 In one specimen from Hammersmith, I found that there were more 

 than 20,000 per grain ; and the average number at Erith in August, 

 1882, was above 7000, which is equivalent to aboiit 200,000 per 

 gallon of water at half-ebb, from the surface to the bottom. This 

 enormous number must represent a very large amount of sewage 

 material consumed as food ; and though, as in the case of larger 

 animals, a considerable part of their escrements no doubt consists of 

 organic matter capable of putrefaction, yet there can be no less doubt 

 that the amount entirely consumed in the life-processes of the animals 

 is also great. 



As named above, I kept Cyclops alive for many months by feeding 

 them on human excrement. It is thus easy to understand why, when 

 they abound in the Thames, the relative amount of human excrement 

 is very considerably less than in the winter, when their number must 

 be much smaller. 



We thus appear to be led to the conclusion that when the amount 

 of sewage discharged into a river is not too great, it furnishes food 

 for a vast number of animals, which perform a most important part 

 in removing it. On the contrary, if the discharge be too great, it may 

 be injurious to them, and this process of purification may cease. 

 Possibly this explains why in certain cases a river which is usually 

 unobjectionable may occasionally become offensive. It also seems to 

 make it clear that the discharge of rather too much sewage may 

 produce relatively very great and objectionable results. 



Though such comparatively large animals as entomostraca may 

 remove much putrefiable matter from a river, we cannot suppose that, 

 except incidentally, they remove such very minute objects as disease 

 germs, but it would be a subject well worthy of investigation to 

 ascertain whether the more minute infusoria can, and do consume 

 such germs as a portion of their food. If so, we should be able 

 to understand how living bodies, which could resist any purely 

 chemical action likely to be met with in a river, could be destroyed 

 by the digestive process of minute animals. Hitherto I have had 

 no opportunity for examining this question critically, but have been 

 able to learn certain facts which, at all events, show that it is well 

 worthy of further examination. It is only during the last month that 

 I have paid special attention to the number of the larger infusoria, and 

 various other animals of similar type, met with per gallon in the 

 waters of rivers and the sea, which can be seen and counted by means 

 of a low magnifying power. At low water in the Medway above 

 Chatham, in the first half of June, the average number per gallon has 

 been about 7000, but sometimes as many as 16,000. Their average 

 size was about 1/1000 in. Possibly the number of still more 



