Order 1. =Volvocaceæ.= The individuals in this order are either uni- or multicellular, and during the essential part of their life are free-swimming organisms. They are generally encased in a mucilaginous envelope, through which 2–6 cilia project from every cell. The vegetative reproduction takes place by the division of all, or a few, of the cells of the individual; in some a palmella-stage is found in addition. The sexual reproduction takes place by isogamous or oogamous fertilisation. The Volvocaceæ may be considered to include the original forms of the Chlorophyceæ, because, among other reasons, the motile stage is here the most prominent; they also form the connecting link between the animal Flagellata, and forms intermediate to the _Syngeneticæ_ may perhaps be found amongst them. Three series of green Algæ may be supposed to have taken their origin from the Volvocaceæ: CONJUGATEÆ (_Desmidiaceæ_) which have lost the swarming stage, but whose conjugation is the nearest to the fertilisation in _Chlamydomonas pulvisculus_: the PROTOCOCCACEÆ in which the vegetative divisions have disappeared, while the swarming stage continues to be present, though of shorter duration; and TETRASPORACEÆ, in which the vegetative divisions are more prominent, whilst the swarming stage is less so. A. UNICELLULAR INDIVIDUALS. The principle genera are: _Chlamydomonas_, _Sphærella_, _Phacotus_.--_Sphærella nivalis_ is the Alga which produces the phenomenon of “Red Snow,” well known on high mountains and on ice and snow fields in the polar regions. The red colouring matter which appears in this and other green Algæ, especially in the resting cells, is produced by the alteration of the chlorophyll. _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. [Illustration: FIG. 44--_Gonium pectorale._] [Illustration: FIG. 45.--_Pandorina morum._]