244 THE MICROSCOPICAL NEWS. 
a most clear manner, changed conditions in the water, the dis- 
charge of a certain amount of sewage being indicated by an 
increase in the total number per gallon, or by an alteration in the 
relative numbers of the different kinds, or by both. All my 
remarks apply to the warm part of the year, and not to winter. 
It is known that entomostraca will eat dead animal matter, 
though probably not entirely dependent on it. I have myself 
prove1 that they may be kept alive for many months by feeding 
them on human excrement, though they soon died without it. If 
the amount of food in any water is small, not many of such animals 
can obtain sufficient; but, if it be abundant, they may multiply 
rapidly, since it is asserted that in one season a single- female 
cyclops may give rise to no less than four thousand millions of 
young. In stagnant muddy ponds, where food abounds, I have 
found an average of 200 per gallon. In the case of fairly pure 
rivers the total number of free-swimming animals is not more than 
one per gallon. I, however, found that where what may be called 
sewage was discharged into such water the number per gallon rose 
to twenty-seven, and the per-centage relationships between the 
different groups of entomostraca were greatly changed. In the 
Thames at Crossness, at low water, the number was about six per 
gallon, which fell to three or four at Erith, and was reduced to less 
than one at Greenhithe. 
There is, however, a very decided limit to the increase of ento- 
mostraca when the water of a river is rendered very impure by the 
discharge of too much sewage, probably because oxygen is defi- 
cient, and free sulphide of hydrogen present. Such water is often 
characterised by the great number of worm-like larvze of insects. 
Thus, in the Don, below Sheffield, in summer, I found the number, 
per gallon, of entomostraca only about one-third of what it is in 
pure waters ; whilst, on the contrary, the number of worm-like 
larvee was 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 the excre- 
ments of entomostraca in the recent mud of such rivers as the 
Thames is most surprising. In one specimen, from Hammer- 
smith, I found that there were more than 20,000 per gallon; and 
the average number at Erith, in August, in 1882, was above 7,000, 
which is equivalent to about 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 consider- 
able part of their excrements no doubt consists of organic matter 
capable of putrefaction, yet there can be no less doubt that the 
