=One-celled Animals.=—There are countless millions of animals and plants the existence of which was not suspected until the invention of the microscope several centuries ago. They are one-celled, and hence microscopic in size. It is believed that the large animals and plants are descended from one-celled animals and plants. In fact, each individual plant or animal begins life as a single cell, called an egg cell, and forms its organs by the subdivision of the egg cell into many cells. An egg cell is shown in Fig. 6, and the first stages in the development of an egg cell are shown in Fig. 7. [Illustration: FIG. 6.—Egg cell of mammal with yolk. ] [Illustration: FIG. 7.—Egg cell subdivides into many cells forming a sphere (morula) containing a liquid. A dimple forms and deepens to form the next stage (gastrula). ] The animals to be studied in the first chapter are _one-celled animals_. To understand them we must learn how they eat, breathe, feel, and move. They are called _Protozoans_ (Greek _protos_, first, and _zoon_). All other animals are composed of many cells and are called _Metazoans_ (Greek _meta_, beyond or after). The cells composing the mucous membrane in man are shown in Fig. 8. The cellular structure of the leaf of a many-celled plant is illustrated in Fig. 1. =Method of Classifying Animals.=—The various animals display differences more or less marked. The question arises, are not some of them more closely related than others? We conclude that they are, since the difference between some animals is very slight, while the difference between others is quite marked. [Illustration: FIG. 8.—MUCOUS MEMBRANE formed of one layer of cells. A few cells secrete mucus. ] To show _the different steps in classifying_ an animal, we will take an example,—the cow. Even little children learn to recognize a cow, although individual cows differ somewhat in form, size, colour, etc. The varieties of cows, such as short-horn, Jersey, etc., all form one _species_ of animals, having the scientific name _taurus_. Let us include in a larger group the animals closest akin to a cow. We see a cat, a bison, and a dog; rejecting the cat and the dog, we see that the bison has horns, hoofs, and other similarities. We include it with the cow in a _genus_ called _Bos_, calling the cow Bos taurus, and the bison, Bos bison. The sacred cow of India (Bos indicus) is so like the cow and the buffalo as also to belong to the genus Bos. Why is not the camel, which, like Bos bison, has a hump, placed in the genus Bos? The Old World buffaloes,—most abundant in Africa and India,—the antelopes, sheep, goats, and several other genera are placed with the genus Bos in a _family_ called the _hollow-horned animals_.