Thus

juice. From the stomach it is made to enter the intestine, and is further acted upon by fluids from the liver, the pancreas, and the glands of the intestines themselves. treated it becomes changed from an insoluble state into a fluid which readily penetrates the coats of the digestive tract.

a.o.

p.v.

pa

\.vr.v.

V.

l.ex.

FIG. 2.-Diagram of heart and bloodvessels of the squirrel or other nammal. a.o., aorta; h, vessels of head; l.a., left auricle; l.ex.,

vessels of lower extremities; lg., lung; 1.v., left ventricle; p.a., pulmonary artery; p.v., pulmo

nary vein; r.a., right auricle;

r.v., right ventricle; v., vessels of

viscera. Arteries are represented

by heavy walls.

Many of the organs of the body are placed at a considerable distance from the food as it comes through the coats of the stomach and intestine. In order to supply them with the necessary nourishment a distributing apparatus is required. This is the office performed by the circulatory system, for as rapidly as the food penetrates the walls of the digestive tract it enters the blood, and by the beating of the heart is driven to all parts of the body, which are thus continually kept in a state of repair. The blood serves also to remove waste substances from the various structures or organs of the animal body and to transfer them to the kidneys, skin, or lungs, which effect their removal from the body.

4. Muscular and nervous systems.-Owing to the fact that animals, as a rule, are compelled

to move about in search of food, we find two highly developed systems, the muscular and nervous, which are absent

in the plants. The first of these, constituting what is usually known as the lean meat, is a relatively complex system of organs, differing widely according to the work performed. In the higher animals-the squirrel, for example-there are not less than five hundred muscles, which are under the control of the nervous system.

The nervous system consists of the brain and spinal cord, which in the squirrel are concealed and protected

FIG. 3.-Skeleton of squirrel, showing its relation to the body.

within the skull and back-bone. From them many nerves pass outward to the muscles, and as many pass inward from the eye, ear, nose, tongue, or skin. By the action of these sense-organs the animal determines the nature of its surroundings, detects its food, recognizes the presence of its enemies, and is thus able to direct its movements to the greatest advantage.

5. Multiplication of animals.-The organs thus far considered serve to perpetuate the animal as an individual; but some provision must also be made for the continuance of the race. In the economy of nature each animal before

its death should leave offspring to take the place of the parent when it falls from the ranks. This is effected in various ways. In some of the simpler animals the body may divide into two equal parts, each of which becomes a complete individual. In other cases the animal detaches a relatively small portion of its body, much as a gardener cuts a slip from a plant, and this likewise develops into a new organism. In the greater number of animals, very clearly illustrated by the birds, eggs are produced which under favorable conditions develop into an organism resembling the parents.

6. Summary.-Animals are thus seen to lead active, busy lives, collecting food, avoiding enemies, and producing and 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 elabo ration. 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.

CHAPTER II

THE CELL AND PROTOPLASM

7. Cells.-If we examine very carefully the different parts of a squirrel 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.

8. 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 squirrel, for example, we have already noted the presence of various organs for carrying on different functions, such as those of digestion, circulation, and respiration; and, further, the cells composing these various parts have been modified in accordance with the duties they have to perform. In the muscles the cells are long and slender (Fig. 4, 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. 4, E, F, G).

The cells of the blood present another type (Fig. 4, 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.

9. 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 m.m. (robo and

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.

10. A typical cell.—Some cell, for example that of the liver (Fig. 4, A), 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 jellylike 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