Phacotus lenticularis has an outer covering incrusted with lime, which, at death, or after division, opens out into two halves. Species may be found among Chlamydomonas, in which conjugation takes place between gametes of similar size without cell-wall, but in C. pulvisculus conjugation takes place between male and female aplanogametes which are surrounded by a mucilaginous envelope.
Fig. 44.—Gonium pectorale.
Fig. 45.—Pandorina morum.
B. Multicellular Individuals. The most important genera are Gonium, Stephanosphæra, Pandorina, Eudorina, Volvox.—Gonium has 4 or 16 cells arranged in a definite pattern in a flat plate (Fig. [44]). Pandorina (Fig. [45]), has 16 cells arranged in a sphere (Fig. [45] A). The vegetative reproduction takes place in this way: each cell, after having rounded off, and after the withdrawal of the cilia, divides itself into 16 new ones (Fig. [45] B), each forming a new individual, which soon grows to the size of the mother-individual. It was in this Alga that the conjugation of self-motile gametes was first discovered by Pringsheim, 1869. When conjugation is about to take place, each cell divides into sixteen, as in vegetative reproduction, but the 16 × 16 cells all separate from one another (Fig. [45] C, female gametes, and D, male gametes), and swarm solitarily in the water. The male are, most frequently, smaller than the female, but otherwise they are exactly alike; they are more or less pear-shaped, with a colourless anterior end, 2 cilia, a red “eye-spot,” etc. After swarming for some time they approach each other, two and two, generally a large and a smaller one, and come into contact at their colourless end; in a few moments they coalesce and become one cell (Fig. [45] E, F), this has at first a large colourless anterior end, 4 cilia, and 2 “eye-spots” (Fig. [45] G), but these soon disappear and the cell becomes uniformly dark-green and spherical, and surrounds itself with a thick cell-wall, losing at the same time its power of motion: the zygote (Fig. [45] H) is formed, and becomes later on a deep red colour. On the germination of the zygote, the protoplasmic cell-contents burst open the wall (Fig. [45] J), and emerge as a large swarmspore (Fig. [45] K) which divides into 16 cells, and the first small individual is formed (Fig. [45] L, M).
Eudorina is like Pandorina in structure, but stands somewhat higher, since the contrast between the conjugating sexual cells is greater, the female one being a motionless oosphere.
Fig. 46.—Volvox globator, sexual individual: a antheridia which have formed spermatozoids; b oogonia.
The highest stage of development is found in Volvox (Fig. [46]). The cells are here arranged on the circumference of a sphere, and enclose a cavity filled with mucilage. The number of these cells may vary from 200–22,000, of which the majority are vegetative and not reproductive, but some become large, motionless oospheres (Fig. [46] b); others, which may appear as solitary individuals, divide and form disc-shaped masses of from 8–256 small spermatozoids (Fig. [46] a). After the oosphere has been fertilised by these, the oospore surrounds itself by a thick, sometimes thorny cell-wall, and on germination becomes a new individual of few cells. A few cells conspicuous by their larger size may be found (1–9, but generally 8) in certain individuals, and these provide the vegetative reproduction, each forming by division a new individual.