But this is a common mistake, as you will soon find out for yourselves. Water such as is commonly used for drinking purposes, whether it comes from a well, spring, river, or pond, contains but little animal or vegetable life in proportion to its quantity; you may place drop after drop under the microscope without finding anything visible, and you can only tell what is in it by filtering a great deal of it. Water standing in ditches or pools for a long time, becomes full of growth of various kinds, and is then so discolored and slimy that no one would think of drinking it. [Illustration: CARAPACE OF ANURAE STIPITATA.] Let us return to the little bottle which you filled with Cochituate filterings last month. Take a little from the bottom with your dipping-tube; put it in the live box and examine it with a half-inch objective. You will see many forms that are strange to you, and we will suppose that the first is that of one of the rotifers. These little creatures are called by this name because of two Latin words meaning wheel-carriers, for on their heads they have an arrangement which looks like a wheel, sometimes in rapid motion. [Illustration: FLOSCULARIA ORNATA.] The most common kind is called _Rotifer vulgaris_ (_fig. 1_), and is a very interesting and elastic being. Sometimes he is gloomy and draws himself in so that he looks like a ball; then he will stretch out full length, and opening his wheel, shoot through the water with great speed. Again he will attach his tail to some fixed object, and by the aid of his wheel draw a rapid current of water through his mouth; it is thus that you can best observe him, and by and by you will discover that the apparent wheel is only a result of the rapid sweeping movement of the long hairs or cilia which fringe the opening in the top of the head. Through this opening the water passes, the rotifer gathers his food from the current, and the food passes into the mastax, where it is ground by the masticating apparatus, which is easily seen in motion. [Illustration: DINOBRYON TORTULARIA.] There are several different rotifers found in Cochituate water; among them the most common is the _Anurae Stipitata_. (_Fig. 2._) It is like a turtle, with a shell, or carapace, beautifully ornamented. You will see plenty of these empty shells, and sometimes you will find one inhabited, when you will see that the creature has a bright red eye, and several bundles of cilia, in front of the projecting spires. One of the families of the rotifers is called _Floscularia_, because it resembles a flower; it is attached at the base to small plants, or algae, and occupies a sheath so transparent that it is hardly visible. One species is occasionally found in the Cochituate, the _Floscularia ornata_. (_Fig. 3._) It is a beautiful object, with its elongated radiating cilia, which remain quiet, and do not vibrate. The specimen figured has three eggs attached to its stem.