Some very curious and complicated changes take place in the lowest of the animals, very similar to those in the lowest vegetables; and what has been called the life-history of these beings, often embraces a great number of forms before the circle of their metamorphoses comes round to the starting-point. Some of these phases of existence are quite different from those going before and after them, and as the vegetable free-cell at one time is capable of motion, it has long been mistaken for an active living animalcule (the Protococcus pluvialis, &c.), and there can be no doubt but that it will be found that many of the lower forms of animated life described by Ehrenberg and others, and still considered as such, will prove to be merely different stages of the metamorphosis of the same protozoon; or it may be not an animal at all, but a simple vegetable or protophyte. [Illustration: FIG. 2.--VOLVOX GLOBATOR.] Dr. Carpenter says, "It is quite certain that the Desmidiaceæ, like the confervoid plants in general, grow at the expense of the inorganic elements which surround them, instead of depending upon other living beings for their subsistence, and that they decompose carbonic acid and give off oxygen under the influence of sunlight. They have the power of generating from these materials the organic compounds which they require for their own development, and these are such as are formed by other undoubted protophytes, as is proved by the application of the appropriate tests." These "Desmidiaceæ" were long considered to belong to the animalculæ, as were the "Diatomaceæ," "Volvox globator" (fig. 2), &c. [Illustration: FIG. 3.--WHEEL ANIMALCULE (Rotifer vulgaris).]