with a small amount of organic matter. On the other hand, water which arrives after a journey overland generally brings with it more or less dirt and débris. In the stream under consideration, contributions of a large volume were received from the underdrains of artificial sand fields upon whose surface had been poured the Brockton city sewage. A strange and heavy burden was thus laid upon a quiet stream hitherto unpolluted. The first thing that happened was a mixture of two very unlike waters, that of the brook and that of the underdrains. This little creek of pure water with a summer flow of only one half million gallons per day, bright, clear, sweet smelling, containing little carbonic acid and few bacteria, poor in nitrates, poor in organic matter but rich in dissolved oxygen, becomes charged with a daily flow of two million gallons of water poor in oxygen, laden with carbon dioxide, burdened with bacteria, rich in plant foods, malodorous and full of broken-down organic compounds of uncertain composition and dubious ancestry. Forthwith various fermentations and other mysterious biological operations begin.

Above the Brockton sewage beds the brook has a clean, sandy or peaty bottom and the usual variety of plants and animals to be found in a clear New England brook. As the first effluent drains pour in their contribution, the bottom of the stream in summer becomes brown with a gelatinous growth of the iron bacterium, Crenothrix, which may occur to a depth of two or three inches over the whole bed of the stream. As the proportion of polluted water is greater, the bottom becomes black with organic material which has settled out from solution and suspension together with silica and other inorganic material forming a sort of "pollution carpet" or "false bottom" in which biological activity is very intense, and where chemical changes are rapidly taking place. There are many bacteria in the polluted water but the number in this bottom mud is much greater, while protozoa, rotifers, worms and insect larvæ also abound. The sides of the stream in many places are gray with furry growths of the colonial protozoa, Carchesium.

Throughout the regions where this false bottom is present —and it extends for three fourths of a mile below the filter beds -there may be seen large patches of the red worm, Tubifex tubifex. The countless individuals making up these colonies. of bristle worms, so typical of pollution, remain with their heads in their burrows, the tails, like a red flag of pollution, waving in the water above them in search of oxygen. They quickly disappear into their holes at the slightest disturbance

more abundant in the stream below or above the area of greatest pollution. Snapping turtles, water snakes and the green frog, Rana clamata, are frequently seen and the valley of the brook contains an abundance of bird life including wild ducks, crows, robins, bobolinks, blackbirds, grackles and sparrows.

By far the most conspicuous of the insect larvæ in this region of abundant food material is the "blood worm" or larva of the midge, Chironomus decorus, which is one of the most efficient scavengers of this type of stream bottom. The mature insect is about the size of the mosquito and in appearance bears a close resemblance, although fortunately its habits are much less objectionable. In this species there is no irresistible impulse to gorge itself with blood and usually the mature form never feeds. The eggs are laid in rows in a gelatinous sack which is attached to some object at the surface of flowing water. Each sack contains 1,200 to 1,500 eggs and so numerous are they that a partly submerged railroad tie was found to furnish a hatching place for over 300,000 midges. The larva is hatched as a white wiggler nearly 1 mm. long which soon settles to the bottom and spends most of its time in eating, growing in three or four days to one fourth of an inch in length and taking on a pink color which soon develops into a brilliant red. At this time the larva begins to make a case for itself by gluing together the material about it with a gelatinous substance from its salivary glands. In many places on the muddy and soft stream bottom, these little mounds, each with an opening at the top, number four or five hundred per square foot. A 30 c.c. sample of bottom mud collected to a depth of 2" in August, 1914, contained 130 of these larvæ. The larval period lasts from several days to several weeks depending upon the temperature, then follows a brief pupal stage in which the insect undergoes metamorphosis and at the end of which the pupa, which is also active, rises to the surface. The pupal case is split along the back, the imago emerges, rests for a minute on the floating pupal case and then flies away. If the good fortune by which it escaped fish and predatory insects continues and it is able to avoid the numerous dragon flies and other enemies for a day or two it reproduces and dies. From the omnivorous habits of the larvæ, their value as food for fish, and the number of mature insects which leave the water entirely, it is obvious that Chironomus is an important factor in the removal of organic matter.

Of the great variety of insect larvæ found in this region, mention may here be made of only one other, the larva of the

Midsummer view of the Stream from near Station 2. This shows the part of the stream which is receiving pollution from the sand sewage-filter beds. Note that the stream is somewhat swifter here and the vegetation on the banks is most luxuriant. From photo taken July 21, 1915.

club-footed gnat, Ptychoptera clavipes. These brown airbreathing larvæ have the same habitat as do the blood worms, although they are not as numerous or as important from the standpoint of stream purification. The first generation appears about the middle of March and from that time they are abundant until late fall. Both the larval and pupal stages are

A corresponding view on January 26, 1916, showing where some of the sewage effluent drains enter the stream, as well as the appearance of the banks in winter and the increased volume of water.

to be found in relatively shallow and quiet water with the elongated breathing tubes reaching to the surface.

It must not be concluded that plant life is absent from this region. To be sure the variety of higher plants disappears at the point of pollution, but one by one these plants reappear as the pollution is reduced, so that the region three quarters of a mile below the filter beds contains the rankest growth of water plants to be found in any part of the stream. Throughout the summer and until late fall they choke the stream in this region, reducing the velocity of the current, furnishing shelter to a wide variety of smaller animal and plant forms, supplying oxygen to the stream, and acting as a contact filter upon which may settle out suspended and colloidal substances. So efficient is this filter that attempts to measure the velocity of stream flow by adding a coal-tar dye to the water were completely frustrated.

In the masses of these plants as well as upon the stream bottom, there may be found innumerable snails and small crustacea. Upon one occasion fifty snails were gathered by one scoop with the two hands at a point one half a mile below the filter beds. The Isopod, Asellus, which is present in all parts of the stream, is so abundant here that as many as twenty may often be seen upon a square foot of the stream bottom, while a handful of water grass or pond weed may contain twice this number. Smaller crustacea, the water fleas, are also abundant, their presence being correlated with that of the simple green algæ. In a pool a short distance below the filter beds the water is green in summer with Chlamydomonas and Euglena. At such times the Daphnia-like form, Simocephalus, is so abundant that even the surface water contains more than one thousand per liter. In all these the digestive tract is filled with the green flagellates which appear to constitute the chief food. Special chemical tests show that there is a daily seesaw between the oxygen dissolved in the water and CO, under these conditions. The former goes up continually through the daytime under the stimulation of sunlight upon the chlorophyll-bearing flagellates, and the latter goes up continually through the night because of animal activity, the total volume of oxygen in the two gases remaining practically constant. In the fall when these water fleas are most abundant myriads of Hydra are found throughout the higher plants.

This in brief is the picture of the stream as we find it through the summer months, when it teems with an activity so