Fig. 10.—Stentor sp.; a protozoan
which may be fixed, like Vorticella,
or free-swimming, at will, and
which has the nucleus in the shape
of a string or chain of bead-like
bodies. The figure shows a single
individual as it appeared when fixed,
with elongate, stalked body, and as
it appeared when swimming about
with contracted body. (From life.)

There is no stagnant pool, no water standing exposed in watering-trough or barrel which does not contain thousands of individuals of the one-celled animals. And in any such stagnant water there may always be found several or many different kinds or species. A drop of this water examined with the compound microscope will prove to be a tiny world (all an ocean) with most of its animals and plants one-celled in structure. A few many-celled animals will be found in it preying on the one-celled ones. There are sudden and violent deaths here, and births (by fission of the parent) and active locomotion and food-getting and growth and all of the businesses and functions of life which we are accustomed to see in the more familiar world of larger animals.

Marine Protozoa.—One usually thinks of the ocean as the home of the whales and the seals and the sea-lions, and of the countless fishes, the cod, and the herring, and the mackerel. Those who have been on the seashore will recall the sea-urchins and starfishes and the sea-anemones which live in the tide-pools. On the beach there are the innumerable shells, too, each representing an animal which has lived in the ocean. But more abundant than all of these, and in one way more important than all, are the myriads of the marine Protozoa.

Although the water at the surface of the ocean appears clear and on superficial examination seems to contain no animals, yet in certain parts of the ocean (especially in the southern seas) a microscopical examination of this water shows it to be swarming with Protozoa. And not only is the water just at the surface inhabited by one-celled animals, but they can be found in all the water from the surface to a great depth below it. In a pint of this ocean-water there may be millions of these minute animals. In the oceans of the world the number of them is inconceivable. And it is necessary that these Protozoa exist in such great numbers, for they and the marine one-celled plants (Protophyta) supply directly or indirectly the food for all the other animals of the ocean.

Among all these ocean Protozoa none are more interesting than those belonging to the two orders Foraminifera (fig. [11]) and Radiolaria. The many kinds belonging to these orders secrete a tiny shell (of lime in the Foraminifera, of silica in the Radiolaria) which encloses most of the one-celled body. These minute shells present a great variety of shape and pattern, many being of the most exquisite symmetry and beauty. The shells are perforated by many small holes through which project long, delicate, protoplasmic pseudopodia. These fine pseudopodia often interlace and fuse 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 protoplasm—part of the body protoplasm of the Protozoan—surrounding the cell externally.

Fig. 11.—Rosalina varians, a marine protozoan (Foraminifera) with calcareous shell. (After Schultze.)

When these tiny animals die their hard shells sink to the bottom of the ocean, and accumulate slowly, in inconceivable numbers, until they form a thick bed on the ocean floor. Large areas of the bottom of the Atlantic Ocean are covered with this slimy ooze, called Foraminifera ooze or Radiolaria ooze, depending on the kinds of animals which have formed it. Nor is it only in present times that there has been a forming of such beds by the marine Protozoa. All over the world there are thick rock strata composed almost exclusively of the fossil shells of these simplest animals. The chalk-beds and cliffs of England, and of France, Greece, Spain, and America, were made by Foraminifera. Where now is land were once oceans the bottoms of which have been gradually lifted above the water's surface. Similarly the rock called Tripoli found in Sicily and the Barbadoes earth from the island of Barbadoes are composed of the shells of ancient Radiolaria.