or protoplasm, as it is termed, is a living substance. know of no case where life exists apart from protoplasm, and for this reason the latter is frequently termed the physical basis of life.

In addition to the features already described, the protoplasm of every perfect cell is modified upon the interior to

FIG. 4.-Different types of cells composing the body of the squirrel or other highly developed animal. A, liver-cell; f, food materials; n, nucleus. B, blood-cell. C, nerve-cell with small part of its fiber. D, muscle fiber. E, cells lining the body cavity. F, lining of the windpipe. G, section through the skin. Highly magnified.

form a well-defined spherical mass known as the nucleus. Other structures are known to occur in the typical cell. Experiment shows that the nucleus and cell protoplasm are absolutely indispensable, whatever their size and shape, and

therefore we are at present justified in defining the cell as a small mass of protoplasm enclosing a nucleus.

11. Structure of protoplasm.—When seen under a lens of moderate power protoplasm gives no indication of any definite structure, and even with the highest magnification it presents appearances which are not clearly understood. According to the commonly accepted view, it consists of two portions, one, the firmer, forming an excessively delicate meshwork (Fig. 4, A) enclosing in its cavities. the second more fluid part. Therefore, when highly magnified, the appearance would be essentially like a sponge fully saturated with water; but it should be remembered that in the protoplasm the sponge work, and possibly the fluid part, is living, and that both are transparent.

There are reasons for thinking that the structure and the composition of protoplasm may change somewhat under certain circumstances. It certainly is not everywhere alike, for that of one animal must differ from that of another, and different parts, such as the liver and brain, of the same form must be unlike. These differences, however, are minor when compared to the resemblances, for, as we shall see, this living substance, wherever it exists, carries on the processes of waste, repair, growth, sensation, contraction, and the reproduction of its kind.

CHAPTER III

THE PROTOZOA

12. Single-celled and many-celled animals.-In almost every portion of the globe there are multitudes of animals whose body consists of but a single cell; while those forms. more familiar to us, and usually of comparatively large size and higher development, such as sponges, insects, fishes, birds, and man himself, are composed of a multitude of cells. For this reason the animal kingdom has been divided into two great subdivisions, the Protozoa including all unicellular forms and the Metazoa embracing those of many cells.

13. Single-celled animals.-The division of the Protozoa comprises a host of animals, usually of microscopic size, inhabiting fresh or salt water or damp localities on land in nearly every portion of the globe. The greater number wage their little, though fierce, wars on one another without attracting much attention; others, in the sharp struggle, have been compelled to live upon or within the bodies of other animals, and many have become notorious because of the diseases they produce under such circumstances. A few are in large measure responsible for the phosphorescence of the sea; and still others have long been favorite objects of study because of their marvelous beauty. Adapted for living under diverse conditions, the bodily form differs greatly, and yet all conform to three or four principal types, of which we may gain a good idea from the study of a few representative forms.

14. The Amoeba.-Among the simplest one-celled animals living in the ooze at the bottom of nearly every freshwater stream or pond is the Amaba (Fig. 5, A), whose body is barely visible to the unaided eye. Under the microscope

FIG. 5.-A, the Amaba, highly magnified, showing c. v., pulsating vacuole; f, food particle; n, nucleus. The arrows show the direction of movement. B, shape of same individual 30 seconds later. C, an amoeba-like animal (Difflugia) partially enclosed in a shell. D, an Amaba in the process of division. E, Gromia, another shelled protozoan (after SCHULZE).

it is seen to consist of an irregular, jelly-like mass of protoplasm totally destitute of a cell wall. Unlike those animals with which we are familiar, the body constantly changes its shape. A rounded bud-like projection will be seen to appear on one side of the body and the protoplasm of adjacent regions flows into it, thereby increasing its extent. Similar projections at the opposite end of the cell are withdrawn, and their substance may flow into the newly formed lobe, which gradually swells in size and pushes forward. Thus, by constantly advancing the front part of the body and

retracting the hinder portion, the cell glides or flows along from place to place.

Upon meeting with any of the smaller organisms upon which it lives, projections from the body are put out which gradually flow around the prey and it becomes pressed into the interior of the cell. The process is not unlike pushing a grain of sand into a bit of jelly. There is no mouth. Any point on the surface serves for the reception of food. Oxygen gas also is taken into the body all over the surface, and wastes and indigestible material are cast out at any point. Nothing exists in these simple forms comparable to the complex systems of organs that carry on these processes in the squirrel.

The bodily size of animals is limited, and to this general rule the Amaba is no exception, for upon gaining a certain size, the nucleus divides into two exactly similar portions, and very soon afterward the rest of the body separates into two independent masses of equal size (Fig. 5, D), each of which, when entirely free, contains a nucleus. In this way two daughter amoebæ are formed possessing exactly the characters of the parent save that they are of smaller size; but it is usually not long before they reach their limit of growth, when division occurs again, and so on, generation after generation.

It not infrequently happens, however, that the pond or stream, in which the Amaba and other Protozoa live, dries up for a portion of the year. In such an event the body assumes a spherical shape, develops a firm, horn-like membrane about itself, and thus encysted it withstands the summer's heat and dryness and may be transported by the wind, or otherwise, over great distances. When the conditions again become favorable the wall ruptures and the Amaba emerges to repeat its life processes.

15. Some relatives of the Amoeba.-All amoeba-like forms, to the number of perhaps a thousand species, possess this same method of locomotion, but many present some inter