Of the Chlamydomonadidae, _Sphaerella_ (_Haematococcus_, Ag.) _pluvialis_ (Fig. 43), and _S. nivalis_, in which the green is masked by red pigment, give rise to the phenomena of "red snow" and "bloody rain." The type genus, _Chlamydomonas_, is remarkable for the variations from species to species in the character and behaviour of the gametes. Sometimes they are equal, at other times of two sizes. In some species they fuse immediately on approximation, in the naked active state; in others, they encyst on approaching, and unite by the emission of a fertilising tube, {126}as in the Algal Conjugatae. _Zoochlorella_ is symbiotic in green Ciliata (pp. 153 f., 158), Sponges (p. 175), _Hydra_ (p. 256), and Turbellaria (Vol. II. p. 43). [Illustration: FIG. 43.—_Sphaerella pluvialis._ A, motile stage; B, resting stage; C, D, two modes of fission; E, _Sphaerella lacustris_, motile stage. _chr_, Chromatophores; _c.vac_, contractile vacuole; _c.w_, cell-wall; _fl_, flagella; _nu_, nucleus; _nu'_, nucleolus; _pyr_, pyrenoids. (From Parker's _Biology_.)] Of the Volvocidae, _Volvox_ (Fig. 44) is the largest and most conspicuous genus. Its colony forms a globe the size of a pin's head, floating on the surface of ponds, drains, or even puddles or water-barrels freely open to the light. It has what may be called a skeleton of gelatinous matter,[139] condensed towards the surface into a denser layer in which the minute cells are scattered. These have each an eye-spot, a contractile vacuole, and two flagella, by the combined action of which the colony is propelled. Delicate boundary lines in the colonial wall mark out the proper investment of each cell. The cells give off delicate plasmic threads which meet those of their neighbours, and form a bond between them. In that half of the hemisphere which is posterior in swimming, a few (five to eight) larger cells ("macrogonidia" of older writers) are evenly distributed, protruding as they increase in size into the central jelly. These as they grow segment to form a new colony. {127}[Illustration: FIG. 44.—_Volvox globator._ A, entire colony, enclosing several daughter-colonies; B, the same during sexual maturity; C, four zooids in optical section; D^1-D^5, development of parthenogonidium; E, ripe spermogonium; F, sperm; G, ovum; H, oosperm. _a_, Parthenogonidia; _fl_, flagellum; _ov_, ovum; _ovy_, ovaries; _pg_, pigment spot; _sp_, sperms; _Spy_, spermogonia dividing to form sperms. (From Parker's _Biology_, after Cohn and Kirchner.)]