colony eats and breathes and feels for itself; each can and does perform all the processes necessary to keep it alive. When ready to multiply, the sixteen cells of the Gonium colony separate, and each cell becomes the ancestor of a new colony.

15. Pandorina. Another colony usually composed of sixteen cells is Pandorina, but the cells are arranged to form a spherical instead of a plate-like colony (Fig. 13). In Pandorina morum the colony consists of sixteen ovoid cells in a spherical jelly-like mass. Each cell has two flagella, and by the lashing of all the flagella the whole colony moves through the water. Food is taken by any of the cells, is assimilated, and the cells increase in size. When Pandorina is ready to multiply, each cell divides repeatedly until it has formed sixteen daughter cells. The inclosing gelatinous mass which holds the colony together dissolves,

and the daughter colonies become free and swim apart. Each colony soon grows to the size of the original colony. This kind of multiplication or reproduction may be continued for several generations. But it does not go on indefinitely. After a number of these generations has been produced by simple division, the cells of a colony divide each into eight instead of sixteen daughter cells. The daughter cells are not all of the same size, but the difference is hardly noticeable. The eight cells resulting from the repeated division. of one of the original cells separate and swim about independently by means of their flagella. If one of these cells comes near a similar free-swimming cell from another

colony, the two cells conjugate (Fig. 14)—that is, fuse to form a single cell. This new cell formed by the fusion of two, develops a tough enveloping membrane of cellulose and passes into what is called

the "resting stage." That is, the cell remains dormant for a shorter or longer time. It may thus tide over a drought or a winter. It may become dry or be frozen, yet when suitable conditions of moisture or temperature are again present the outer wall breaks and the protoplasm issues as a large freeswimming cell, which soon divides into sixteen daughter cells which constitute a new colony.

A

B

16. Eudorina.-Another colonial protozoan which much resembles Pandorina, but differs from it in one interesting and suggestive thing, is Eudorina. In Eudorina elegans (Fig. 15) the colony is spherical and is composed of sixteen or thirty- c two cells. Each of these cells can become the parent of a new

colony by simple repeated divi- FIG. 14. — Pandorina morum (after sion. But this simple mode of reproduction, just as with Pandorina, can not persist indefinitely. There must be conjugation. But the process of mul

GOEBEL). Three stages in the conjugation and formation of the resting spore. A, two cells just fused; B, the two cells completely fused, but with flagella still persisting; C, the resting spore.

tiplication, which includes conjugation, is different from that process in Pandorina, in that in Eudorina the conju

gating cells are of two distinctly different kinds. When this kind of multiplication is to take place in the case of Eudorina elegans, to choose a common species, some of the cells of a colony divide into sixteen or thirty-two

FIG. 15.-Eudorina elegans. A, a mature colony (from Nature); B, formation of the two kinds of reproductive cells.

from the envelope of the parent cell, remaining for some time united in small bundles. Other cells of the colony do not divide, but increase slightly in size and become spherical in shape. When a bundle of the small cells comes into contact with some of these large spherical cells the bundle breaks up, and conjugation takes place between the small flagellated free-swimming cells and the large non-flagellate spherical cells. Each new cell formed. by the fusion of one of the small and one of the large cells develops a cellulose wall and assumes a resting stage. After a time from each of these resting spores a new colony of sixteen or thirty-two cells is formed by direct, repeated division.

17. Volvox.-Another interesting colonial protozoan is Volvox. The large spherical colonies of Volvox globator

A

(Fig. 16) are composed of several thousand cells, arranged in a single peripheral layer about the hollow center of the ball. The cells are ovoid, and each is provided with two long flagella which project out into the water. The lashing of the thousands of the flagella give the balllike colony a rotary motion. The cells are held together by a jelly-like intercellular substance and are connected with each other by fine protoplasmic threads which extend from the body protoplasm of one cell to the cells surrounding it. When the colony is full grown and ready to reproduce itself certain cells of the colony undergo great changes. Some of them increase in size enormously, having reserve food material stored in them, and they may be called the egg cells of the colony. Reproduction may now occur by simple division of one of these great egg cells into many small cells, all held together in a common envelope. These form a daughter colony which escapes from the

C

B

mother colony and by growth and further division comes to be a new full-sized colony. Or reproduction may occur in another, more complex, way. Certain cells of the colony

divide into bundles of very small, slender cells, each of which is provided with flagella. The remaining cells of the colony (that is, those which have not swollen into egg cells or divided into many-sixty-four to one hundred and twenty-eight-minute, flagellate cells) remain unchanged for a while and finally die. They take absolutely no part in reproducing the colony. One of the minute free-swimming cells fuses with one of the enormous egg cells, the new cell thus formed being a resting spore. From this resting spore a new colony develops by repeated division.

18. Steps toward complexity.-Within the group of Volvocina there are plainly several steps on the way from simplicity of structure to complexity of structure. Gonium, Pandorina, Eudorina, and Volvox form a series proceeding from the simplest animals toward the complex animals. In Gonium the cells composing the colony are all alike in structure, and each one is capable of performing all the processes or functions of life. In Pandorina and Eudorina the cells are at first alike, but there is, as the time for reproduction approaches, a differentiation of structure; the cells of the colony, all of which take part in the process of reproduction, come to be in certain generations of two kinds an inactive large kind which may be called the egg cells, and a small, active, free-swimming kind which seeks out and conjugates with, or, we may say, fertilizes the egg cells. In Volvox there is a new differentiation. Only certain particular and relatively few cells take part in reproducing the colony; most of the cells have given up the power or function of reproduction. These cells, when the time of multiplication comes, simply support the special reproductive cells. They continue to waft the great colony through the water by lashing their flagella; they continue to take in food from the outside. The reproductive cells devote themselves wholly to the business of producing new colonies, of perpetuating the species. And this matter of reproduction is less simple than in the other Volvocina.