You will find other rotifers in the Cochituate, some formed like vases, others with long spires, but all graceful and beautiful. The _Dinobryon Tortularia_ is sometimes very common in this water. In October, 1881, when the taste of the water was very bad, the _Dinobryon_ was very abundant, though we do not know that it had anything to do with the bad taste. You will see by the figure, that it is like a tree, with an individual of the family at the end of every branch. Each one has its own organs of existence, although attached to its brothers by its stem. Each has a bright red eye, and a long slender whip, called a _flagellum_, with which it lashes the water, and when all the _flagellae_ are in motion, the whole tree swims about. The individuals are very small indeed, and it will take your best objective to show the _flagellum_. [Illustration: VORTICELLA NEBULIFERA.] Another tree-like group is that of the _Vorticella_, of which you will sometimes find in the Cochituate, the species _Vorticella nebulifera_. Each animal is at the top of a stem, and this stem has the peculiar property of being able to coil up and draw its head down close to the bottom. This appears to be a defensive movement, for whenever a big ugly creature comes by, down go the whole family so quickly that your eye cannot follow the movement. Sometimes they will all bob down when you tap the stage of the microscope so as to jar them. At a certain period of its life the animal suddenly leaves its stem, and goes swimming about with great speed. VI.--INTERESTING OBJECTS. [Illustration: STEPHANODISCUS NIAGARAE.] The most beautiful of the small _algae_ or water plants are the _Diatomaceae_ and the _Desmidiaceae_, sometimes called for brevity diatoms and desmids. They are remarkable for the geometrical character of their forms, consisting of circles, triangles and polygons of infinite variety. They are very small, and cannot be satisfactorily seen with an objective of less power than a four tenths. The diatoms are found everywhere in both fresh and salt water, but the desmids live only in fresh water. One of the most common diatoms in Cochituate water is the _Stephanodiscus Niagarae_. (_Fig. 1._) It is in shape like a pill box, and its sides, which would be called its top and bottom if it were a pill box, are beautifully ornamented with dots in radiating lines with a ring of spines near the edge. This circle of spines or thorns explains its name, _Stephanodiscus_, from the proto-martyr, Saint Stephen. The name _Niagarae_ is from Niagara River, where it was found. Like all diatoms, it contains when alive a yellowish brown matter with small globules of oil, which is called _endochrome_. The box or shell, called _pustule_, is of silex or quartz, and is therefore almost indestructible; and when the diatom dies, sinks to the bottom of the water. In this way beds of shells of diatoms are sometimes formed of considerable thickness. [Illustration: ASTERIONELLA BLEAKLEYII.]