fine threads interlace when they touch each other, thus forming a sort of protoplasmic network outside of the shell. In some cases there is a complete layer of protoplasmpart of the body protoplasm of the protozoan -surround

FIG. 10.-Polystomella strigillata, one of the Globigerinæ.-After MAX SCHULTZE.

ing the cell externally. The Radiolaria, whose shells are made of silica, possess also a perforated membranous sac called the central capsule, which lies imbedded in the protoplasm, dividing it into two portions, one within and

one outside of the capsule. In the protoplasm inside of the capsule lies the nucleus or nuclei; and from the protoplasm outside of the capsule rise the numerous fine, threadlike pseudopods which project through the apertures in the shell, and enable the animal to swim and to get food.

Most of the myriads of the simplest animals which swarm in the surface waters of the ocean belong to a few kinds of these shell-bearing Globigerinæ and Radiolaria. Large areas of the bottom of the Atlantic Ocean are covered with a slimy gray mud, often of great thickness, which is called globigerina-ooze, because it is made up chiefly of the microscopic shells of Globigerinæ. As death comes to the minute protoplasmic animals their hard shells sink slowly to the bottom, and accumulate in such vast quantities as to form a thick layer on the ocean floor. Nor is it only in present times and in the oceans we know that the Globigerinæ have flourished. All over the world there are thick rock strata which are composed chiefly of the fossilized shells of these simplest animals. Where the strata are made up exclusively of these shells the rock is chalk. Thus are composed the great chalk cliffs of Kent, which gave to England the early name of Albion, and the chalk beds of France and Spain and Greece. The existence of these chalk strata means that where now is land, in earlier geologic times were oceans, and that in the oceans Globigerinæ lived in countless numbers. Dying, their shells accumulated to form thick layers on the sea bottom. In later geologic ages this sea bottom has been uplifted and is now land, far perhaps from any ocean. The chalk strata of the plains of the United States, like those in Kansas, are more than a thousand miles from the sea, and yet they are mainly composed of the fossilized shells of marine Protozoa. Indeed, we are acquainted with more than twice as many fossil species of Globigerinæ as species living at the present time. The ancestors of these Globigerinæ, from which the present Globigerinæ differ but little, can be

traced far back in the geologic history of the world. It is an ancient type of animal structure.

The Radiolaria, too, which live abundantly in the present oceans, especially in the marine waters of the tropical and temperate zones, are found as fossils in the rocks from the time of the coal age on. The siliceous shells of the

Radiolaria sinking to the sea bottom and accumulating there in great masses form a radiolaria-ooze similar to the globigerinæ-ooze; and just as with the Globigerinæ, the remains of the ancient Radiolaria formed thick layers on the floor of the ancient oceans, which have since been uplifted and now form certain rock strata. That kind of rock called Tripoli, found in Sicily, and the Barbados earth from the island of Barbados, both of which are used

as polishing powder, are composed almost exclusively of the siliceous shells of ancient and long-extinct Radiolaria.

10. Antiquity of the Protozoa.-All the animals of the ocean depend upon the marine Protozoa (and the marine Protophyta, or one-celled plants) for food. Either they prey upon these one-celled organisms directly, or they prey upon animals which do prey on these simplest animals. The great zoologist already quoted says: "The food supply of marine animals consists of a few species of microscopic organisms which are inexhaustible and the only source of food for all the inhabitants of the ocean. The supply is primeval as well as inexhaustible, and all the life of the ocean has gradually taken shape in direct dependence upon it." That is, the marine simplest animals are the only marine animals which live independently; they alone can live or could have lived in earlier ages without depending on other animals. They must therefore be the oldest of marine animals. By oldest we mean that their kind appeared earliest in the history of the world. As it is certain that marine life is older than terrestrial life-that is, that the first animals lived in the ocean-it is obvious that the marine Protozoa are the most ancient of animals. This is an important and interesting fact. Zoologists try to find out the relationships and the degrees of antiquity or modernness of the various kinds of animals. We have seen that the Protozoa, those animals which have the simplest body structure and perform the necessary life processes in the simplest way, are the oldest, the first animals. This is just what we would expect.

11. The primitive form. We find among the simplest animals a considerable variety of shape and some manifest variation in habit. But the points of resemblance are far more pronounced than the points of difference, and are of fundamental importance. The composition of the body of one cell, as opposed to the many-celled structure of the bodies of all other animals, is the fact to be most distinctly

emphasized. The shape of this one-celled body varies. With the most primitive or simplest of the "simplest animals," like Amæba, for example, there is no "distinction of ends, sides, or surfaces, such as we are familiar with in in the higher animals. Anterior and posterior ends, right and left sides, dorsal and ventral surfaces are terms which have no meaning in reference to an Amaba, for any part of the animal may go first in locomotion, and when crawling the animal moves along on whatever part of its surface happens to be in contact with foreign bodies." The one shape most often seen among the Protozoa, or most nearly fairly to be called the typical shape, is the spherical or subspherical shape. Why this is so is readily seen. Most of the Protozoa are aquatic and free swimming. They live in a medium, the water, which supports or presses on the body equally on all sides, and the body is not forced to assume any particular form by the environment. The body rests suspended in the water with any part of its surface uppermost or any part undermost. any part of the surface serves equally well in many of the Protozoa for breathing or eating or excreting, it is obvious that the spherical form is the simplest and most convenient shape for such a body. It is interesting to note that the spherical form is the common shape of the egg cell of the higher animals. Each one of the higher, multicellular animals begins life (as we shall find it explained in another chapter of this book) as a single cell, the egg cell, and these egg cells are usually spherical in shape. The full significance of this we need not now attempt to understand, but it is interesting to note that normally the whole body of the simplest animals is a single spherical cell, and that every one of the higher animals, however complex it may become by growth and development, begins life as a single spherical cell.

12. The primitive but successful life.-Living consists of the performing of certain so-called life processes, such as