Fig 26.—Spondylomorum, a small colony of flagellates.
Fig. 27.—Magosphæra, a colonial flagellate.
Our information regarding the early history of this co-operative movement is fragmentary and incomplete, for only an odd species or so seems to survive of the group which we regard as the earliest of multicellular animals. In certain forms which are still essentially unicellular, such as the Spondylomorum shown in Fig. 26, there is a tendency to form smaller or larger cell colonies. When the individual cell divides, the two daughter cells do not separate, but remain somewhat loosely attached to each other, and the process of division without separation continues until a considerable group is produced. From this colony occasional individuals break away and proceed to form new colonies. From such a type it is a comparatively easy step to the Magosphæra described by Haeckel and illustrated in Fig. 27. This consists of a simple ball of ciliated cells, which reproduces by the occasional breaking away of an individual member, which divides and redivides until a new sphere is produced. Unfortunately this animal has only once been discovered, and many hold that it has not been sufficiently investigated. No other of the same type is known.
If we turn to the plant kingdom, however, we find a comparatively common organism of somewhat similar form. This is the Volvox, a plant which consists of some thousands of cells, and reaches a size of about a pin-head. It has the form of a hollow sphere, the wall of which is one cell thick, and the cavity of which contains only water. The cells bear whip-like cilia on their outer surface, by whose means the organism is able to move, swimming by a rotary motion round a definite axis. The individual cells are separated by layers of a gelatinous substance, through which, however, pass connecting strands of protoplasm. The cells, of course, contain the green colouring matter common to plants in general. Distributed among the ordinary cells occur a few that are distinguished by their larger size, and by the fact that they lack cilia. These are the special reproductive members of the colony. When the Volvox reaches maturity, these cells begin to divide, and form new growths which take the form of hollow sacs, which project into the cavity of the parent sphere. Later they separate from the wall of the parent, and begin to move about, in the internal cavity, by means of the cilia which they have developed. Finally the parent breaks up and dies, and the progeny are set free to commence life for themselves.
Fig. 28.—Volvox. A female, showing egg cells.
Fig. 29.—Volvox. Male, showing packets of sperm cells.