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SPONGILLA FLUVIATILIS.

DESMIDIUM SWARTZII. FRONT AND SIDE VIEW.

Another diatom common in Cochituate is Tabellaria Fenestrata, which grows in ribbon-like forms. (Fig. 3.) The desmids resemble the diatoms in the geometrical character of their forms, but they have no shell of silex, and are therefore easily destroyed. They are readily distinguished at sight by the beautiful green color of the contained matter. In many of them there is a curious circulation of small particles, especially in the ends of those of a crescent or new-moon shape. This circulation can only be seen with a high power. Desmids are easily found in ponds and ditches; and there are several species in Cochituate. Among them is Desmidium Swartzii (fig. 4), and Closterium moniliferum. (Fig. 5.) Their beauty depends so much on color that they do not appear to advantage in the figures. You will find in examining the filterings of Cochituate water, many objects which have not been described in these papers, and among them many fragments of green filaments of the small plants belonging to the confervæ and the oscillatoriæ; sometimes you will find small round opaque forms of brown or green color, which are probably spores of plants of a larger growth; sometimes you will see the pollen of pine-trees which has fallen into the water and looks like three small balls fastened together; sometimes, though rarely, you may find one of those curious little creatures called water bears, or tardigrada; and you may be fortunate enough to catch a water spider.

CLOSTERIUM MONILIFERUM.

But you will often see the spiculæ of the sponge, called Spongilla fluviatilis. They look like pins of glass, blunt at one end and pointed at the other, and are sometimes very abundant. You may have heard that this sponge has been considered the source of the occasionally bad taste and smell of Cochituate water. When it is alive, it is not disagreeable, but when it decays it imparts to the water a very unpleasant taste and odor. It certainly is one cause of the bad quality of the water, but whether it is entitled to the sole credit is still open to question.

You can see what it looks like in fig. 6. When alive, it is of a light-green color, but when decayed it becomes brown. It is full of the spiculæ above described, which serve to stiffen it, but it easily crumbles and scatters them through the water.

Though the microscope shows us many beautiful and interesting objects, yet in the present state of our knowledge we cannot ascertain by its use whether the water we examine is harmless or injurious.

VII.—THE BRICKMAKER.

The microscope reveals so many strange odd-looking water creatures and plants that we can easily imagine ourselves transported to some new world. Look at this field of view as seen through the microscope. In the centre stands a brickmaker. He is a queer little animal, and so small that he looks like a mere speck to the naked eye, but through the microscope we see how wonderfully curious and strange a creature he is. He is no idle, lazy fellow. He is instead a most busy mechanic.

Just now he is building a house out of tiny bricks, and he manufactures the bricks himself, making them one at a time, and when one is finished he lays it down carefully by the side of the last, and fastens it firmly in its place with a kind of cement. The bricks are laid in regular tiers one above the other.

We find these brickmakers in still water where various water-plants grow, especially the water-milfoil and bladderwort. They seem to be social beings. They live in large communities, attaching their houses to the stems and leaves of the plants so thickly sometimes that they almost touch one another. They look, to the naked eye, like lines about one eighth of an inch in length. Sometimes they are very thick on the plants in New Jersey ponds.

If you take some of the plants and water, and put them in a bottle, you can carry a large number of the brickmakers home, where you can watch them at your leisure. Take a glass slide which has a little cup-shaped hollow to hold a few drops of water, and put a tiny piece of the plant with the house attached into this hollow and fill it with some of the water from the bottle. Now cover it with a very thin piece of glass and lay it over the stage of the microscope, and it is ready to be looked at and studied. You will look with both eyes, for your microscope is a binocular—one that has two tubes to look through. The size of the objects will depend upon the magnifying power you have chosen.

The first thing you see is a dark, brick-colored, cylinder-shaped house which looks to be about the size of a cigar. The little builder who lives in this house has been disturbed by the means we have taken to make his acquaintance; he has stopped work and gone within. But he is so industrious a fellow that he will not remain within very long. As soon as it is quite still he will probably come to the door of his house, and you will see him thrust out two horns. He will move these horns to the right and left, cautiously feeling all around him. He seems very cautious indeed. But at last he is satisfied that no enemy is near. Now he ventures out. He unfolds his wheels.

These wheels are surrounded with a band of cilia, or flexible hairs, which he can put in rapid motion, making the wheels have the appearance of revolving very fast. This rapid motion of the cilia forms a swift current in the water; and this current brings tiny particles of various things to the little mechanic. Some of these particles he uses for food; of others, he makes brick. They are carried into an opening between the wheels where you can see them revolving very fast until they are gathered into a little round, dark-colored pellet. The particles are probably held together by a sticky secretion made by the builder.

It takes him about three minutes to make a brick. As soon as it is finished, he bends his head over, takes it from its mould between the wheels, and lays it down carefully by the side of the last. Then he raises his head and begins to make another. The tube thus constructed is quite firm and strong. Sometimes when I have found a long tube, I have cut off a portion from the top. This can be done, with care, for the brickmaker drops to the bottom when disturbed. It is very amusing to watch him repair damages and rebuild. Sometimes I have forced one out of his tube, but it always soon died. But though industrious, he is so cautious, or timid, that he is easily frightened, and therefore he is often interrupted in his work. For instance, like some people that we know, he is very afraid of snakes. If a harmless little tiny snake comes wriggling along through the water anywhere near him, he folds his wheels and drops down into his house as quick as a flash. One day a little boy was delighted with the fast-revolving wheels. Suddenly, by and by, he turned toward me with great