prey, when both captor and victim may be destroyed by the expansion of the body. Some fishes die on being taken out of water through the swelling of the air bladder and the bursting of its blood-vessels. If an animal which lives normally on the surface of the earth is taken up a very high mountain or is carried up in a balloon to a great altitude where the pressure of the atmosphere is much less than it is at the earth's surface, serious consequences may ensue, and if too high an altitude is reached death occurs. This death may be in part due to the difficulty in breathing in sufficient oxygen to maintain life, but it is probably chiefly due to the disturbances caused by the removal of the pressure to which the body is accustomed and is structurally adapted to withstand. All living animals are accustomed to live under a certain pressure, and there are evidently limits of maximum or minimum pressure beyond which no animal at present existing can go and remain alive.

But in the case both of temperature and pressure conditions it is easy to conceive that animals might exist which could live under temperature and pressure conditions not included between the minimum and maximum limits of each as determined by animals so existing. But it is impossible to conceive of animals which could live without oxygen or without organic food. The necessities of oxygen and organic food (and water) are the primary or essential conditions for the existence of any animals.

Of course, we might include such conditions, among the primary conditions, as the light and heat of the sun, the action of gravitation, and other physical conditions, without which existence or life of any kind would be impossible on this earth. But we here consider by “primary conditions of animal life" rather those necessities of living animals as opposed to the necessities of living plants. Neither animals nor plants could exist without the sun, whence they derive directly or indirectly all their energy.

8. Cells. If we examine very carefully the different parts of some highly developed animal under the high powers of the microscope we find that they are composed of a multitude of small structures which bear the same relations to the various organs that bricks or stones do to a wall; and if the investigation were continued it would be found that every organism is composed of one or more of these lesser elements which bear the name of cells. In size they vary exceedingly, and their shapes are most diverse, but, despite these differences, it will be seen that all exhibit a certain general resemblance one to the other.

9. Shape of cells.-In many of the simpler organisms the component cells are jelly-like masses of a more or less. spherical form, but as we ascend the scale of life the condition of affairs becomes much more complex. In the muscles the cells are long and slender (Fig. 1, D); those forming the nerves and conveying sensations to and from all parts of the body, like an extensive telegraph system, are excessively delicate and thread-like; in the skin, and lining many cavities of the body, where the cells are united into extensive sheets, they range in shape from high and columnar to flat and scale-like forms (Fig. 1, E, F, G). The cells of the blood present another type (Fig. 1, B); and so we might pass in review other parts of the body, and continue our studies with other groups of animals, always finding new forms dependent upon the part they play in the organism.

10. Size of cells. Also in the matter of size the greatest variations exist. Some of the smallest cells measure less than one micromillimeter ( of an inch) in diameter. Over five hundred million such bodies could be readily stowed away into a hollow sphere the size of the letter beginning this sentence. In a drop of human blood of the same size, between four and five million blood-cells or corpuscles float. And from this extreme all sizes exist up to those with a diameter of 2.5 or 5 c.m. (one or two inches),

as in the case of the hen's or ostrich's egg. On the average a cell will measure between .025 to .031 mm. (1 and 8 of an inch) in diameter, a speck probably invisible to the unaided eye. While the size and external appearance of a cell are seen to be most variable, the internal structures are found to show a striking resemblance throughout. All are constructed upon essentially the same plan. Differences in form and size are superficial, and in passing to a more careful study of one cell we gain a knowledge of the important features of all.

11. A typical cell.-An egg-cell (Fig. 1, A) or some simple one from the liver or skin may be chosen as a good representative of a typical cell. To the naked eye it is barely visible as a minute speck; but under the microscope the appearance is that of so much white of egg, an almost transparent jelly-like mass bearing upon its outer surface a thin structureless membrane that serves to preserve its general shape and also to protect the delicate cell material within. The comparison of the latter substance to egg albumen can be carried no further than the simple physical appearance, for albumen belongs to that great class of substances which are said to be non-living or dead, while the cell material or protoplasm, as it is termed, is a living substance. We 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 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.

12. Structure of protoplasm.-When seen under a glass. 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

FIG. 1.-Different types of cells composing the body of a highly developed animal. A, 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.

meshwork (Fig. 1, 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.

13. Animal functions.-Animals in general lead active, busy lives, collecting food, avoiding enemies, and producing and caring for their young. While the activities of all animals are directed to their own preservation and to the multiplication of their kind, these processes are carried on in the most diverse ways. The manner in which an organ or an organism is made, and the method by which it does its work, are mutually dependent one on the other. As there is an enormous number of species of animals, each differently constructed, there is, accordingly, a very great variety of habits. As we shall see, the lower forms are remarkably simple in their construction, and their mode of existence is correspondingly simple. In the higher types a much greater complexity exists, and their activities are more varied and are characterized by a high degree of elaboration. In every case, the animal, whether high or low, is fitted for some particular haunt, where it may perform its work in its own special way and may lead a successful life of its own characteristic type.