emphasized. The shape of this one-celled body varies. With the most primitive or simplest of the "simplest animals," like Amaba, 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. As 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 eating, breathing, feeling, and multiplying. These processes are performed among the higher animals by various organs, special parts of the body, each of which is fitted to do some one kind of work, to perform some one of these processes. There is a division or assignment of labor here among different parts of the body. Such a division of labor, and special fitting of different parts of the body for special kinds of work does not exist, or exists only in slightest degree among the simplest animals. The Amaba eats or feels or moves with any part of its body; all of the body exposed to the air (air held in the water) breathes; the whole body mass takes part in the process of reproduction. Only very small organisms can live in this simplest way. So all of the Protozoa are minute. When the only part of the body which can absorb oxygen is the simple external surface of a spherical body, the mass of that body must be very small. With any increase in size of the animal the mass of the body increases as the cube of the diameter, while the surface increases only as the square of the diameter. Therefore the part of the body (inside) which requires to be provided with oxygen increases more rapidly than the part (the outside) which absorbs oxygen. Thus this need of oxygen alone is sufficient to determine the limit of size which can be attained by the spherical or subspherical Protozoa. That the simplest animals, despite the lack of organs and the primitive way of performing the life processes, live successfully is evident from their existence in such extraordinary numbers. They outnumber all other animals. Although serving as food for hosts of ocean animals, the marine Protozoa are the most abundant in individuals of all living animals. The conditions of life in the surface waters of the ocean are easy, and a simple structure and simple method of performance of the life processes are wholly adequate for successful life under these conditions. That the character of the body structure of the Protozoa has changed but little since early geologic times is explained by the even, unchanging character of their sur roundings. The oceans of former ages have undoubtedly been essentially like the oceans of to-day-not in extent and position, but in their character of place of habitation for animals. The environment is so simple and uniform that there is little demand for diversity of habits and consequent diversity of body structure. Where life is easy there is no necessity for complex structure or complicated habits of living. So the simplest animals, unseen by us, and so inferior to us in elaborateness of body structure and habit, swarm in countless hordes in all the oceans and rivers and lakes, and live successfully their simple lives. CHAPTER II THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 13. Colonial Protozoa.-When one of the simplest animals multiplies by fission, the halves of the one-celled body separate wholly from each other, move apart, and pursue their lives independently. The original parent cell divides to form two cells, which exist thereafter wholly apart from each other. There are, however, certain simple animals which are classed with the Protozoa, which show an interesting and important difference from the great majority of the simplest animals. These are the so-called colony-forming or colonial Protozoa. These colonial Protozoa belong to a group of organisms called the* Volvocinæ. The simplest of the Volvocinæ are single cells, which live wholly independently and are in structure and habit essentially like the other Protozoa we have studied. They have, however, imbedded in the onecelled body a bit of chlorophyll, the green substance which gives the color to green plants and is so important in their physiology. In this respect they differ from the other Protozoa. Among the other Volvocinæ, however, a few or many cells live together, forming a small colony-that is, * These colonial organisms, the Volvocinæ, are the objects of some contention between botanists and zoologists. The botanists call them plants because they possess a cellulose membrane and green chromatophores, and exhibit the metabolism characteristic of most plants; but most zoologists consider them to be animals belonging to the order Flagellata of the Protozoa. In the latest authoritative text-book of zoology, that of Parker and Haswell (1897), they are so classed. there is formed a group of a few or many cells, each cell having the structure of the simpler unicellular forms. These cells are held together in a gelatinous envelope, and the mass is usually spherical in shape. In most of the colonies each of the cells possesses two or three long, protoplasmic, whiplash-like hairs, called flagella, and by the lashing of these flagella in the water the whole group swims about. 14. Gonium.-If, when one of the simplest animals divided to form two daughter cells, these two cells did not move apart, but remained side by side and each di- B A FIG. 12.-Gonium pectorale (after STEIN). A, colony seen from above; B, colony seen from the side. in the case of those colonial Protozoa belonging to the genus Gonium (Fig. 12). When the mother cell of Gonium divides, the daughter cells do not swim apart, but remain side by side, and by repeated fission, until there are sixteen cells side by side, the colony is formed. Each cell of the |