Chlorophycae - Volvocales - Volvox

Phylum Thallophyta - The Algae - Chlorophycae - Volvocales - Volvox 
In Volvox we have the largest colony of this type which is known. It consists of a hollow sphere or ellipsoid of up to 20,000 cells, forming  peripheral layer within the mucous sheath. The individual cells are of the general chlamydomonad type, with globose, oval or stellate chloroplasts, often with several contractile vacuoles and one or more pyrenoids. The cell walls are thick and gelatinous, often with a firmer middle lamella which forms a polygonal pattern between the cells. The cells of the colony are connected together by means of protoplasmic strands. These may be very slender, but in Volvox globator they are thick processes which are linked together by fine strands passing through the middle lamellae. Each cell is thus very similar to Haematococcus. The whole colony moves and rotates slowly by the concerted action of the flagella, two of which project through the mucus from the anterior end of each cell.
Reproduction in Volvox
Asexual reproduction takes place by the formation of daughter colonies within special cells of the parent colony, which are recognizable by their greatly increased size and are called gonidia. By repeated cell divisions the gonidia produce hollow, spherical colonies, which project into
the interior of the parent sphere, and collectively may nearly fill it. They are eventually liberated by the breaking up of the parent colony.
As these daughter col­onies develop, the anterior or outer ends of the cells are directed inwards towards their respective centres. Shortly before their cells de­velop flagella the daughter colonies turn themselves completely inside out by a folding process which begins from the inner end, and thu restores the correct orienta­tion of the cells.
Sexual reproduction is of advanced type, with well-developed oogamy. The gametes may be pro­duced either in distinct colonies or in the same one, either simultane­ously or, more often, at different times. The antherozoids are biflag­ellate and fusiform, i.e., spindle­shaped. They have pale yellowish-green chloroplasts, and are developed in plates or circular masses formed by the division of the contents of specially enlarged cells. The female gametes or oos­pheres are rounded or flask-shaped and have no flagella. They are contained in much enlarged cells of the colony, which are like the gonidia but larger. The oosphere often de­velops a kind of protrusion or beak, which is probably the point of entry of the antherozoid. After fertiliza­tion a spherical oospore is formed, with a smooth or stellate membrane and brownish-red contents. It germinates to form a single monoploid zoospore, which by division gives rise to a new small colony which undergoes the usual eversion during its development.
In the above examples we see the progressive development of the colonial type from the simple, unicellular Chlamydomonas to the highly complex colony of Volvox, with its very many component cells. We also see the gradual division of labour, from the condition in Pandorina in which all the cells of the colony are alike and able to reproduce, through Eudorina, in which there is a slight division of labour, in as much as the cells of the anterior and posterior ends show a difference in their morphology, to Volvox where there are specialized reproductive cells. We also see a tendency towards co­ordination, in the development of protoplasmic connections between the cells of the colony. We do not know the precise function of these connections, but it seems reasonable to suggest that they may in some way assist in con­trolling the movement of the complex colony of Volvox, and it is significant that the connections are more clearly developed in that type than in Eudorina, and that they have not been demonstrated in Pandorina. Another evolutionary tendency which we may notice in this series is the progressive elaboration of the sex organs. In Chlamydomonas the gametes are usually isogamous, in Pandorina they are anisogamous, while in both Eudorina and Volvox they are clearly differentiated into a small male gamete and a large female gamete which is fertilized without liberation, that is to say, is oogamous.
This Volvox series, as it is called, is interesting therefore, because by means of a few types we can illustrate apparent stages in an evolutionary series; not, it is true, one which progressed very far, for it was apparently a side line culminating in Volvox, but one in which the present-day repre­sentatives of the various stages enable us to see the way in which this evolution took place. Its weakness seems to have been the retention of motility by the whole colony, since there are obvious mechanical limitations to progress along this line, and we shall see that all higher plants have evolved on the basis of retaining motility only for the reproductive cells.