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. Order 2. =Tetrasporaceæ= reproduce both by vegetative divisions and swarmspores, some have also gamete-conjugation. The principal genera are: _Tetraspora_, _Apiocystis_, _Dactylococcus_, _Dictyosphærium_, _Chlorangium_. Order 3. =Chlorosphæraceæ.= _Chlorosphæra._ Order 4. =Pleurococcaceæ.= In this order the swarm-stages and sexual reproduction are entirely absent. Vegetative reproduction by division. The principal genera are: _Pleurococcus_ (Fig. 47), _Scenedesmus_ (Fig. 48), _Raphidium_, _Oocystis_, _Schizochlamys_, _Crucigenia_, _Selenastrum_.--_Pleurococcus vulgaris_ (Fig. 47) is one of the most common Algæ throughout the world, occurring as green coverings on tree-stems, and damp walls, and it is one of the most common lichen-gonidia. [Illustration: FIG. 47.--_Pleurococcus vulgaris._] [Illustration: FIG. 48.--_Scenedesmus quadricauda._] Order 5. =Protococcaceæ.= The cells are motionless, free or affixed on a stalk (_e.g. Characium_, Fig. 49), either separate or loosely bound to one another; they never form multicellular individuals. Multiplication by division is nearly always wanting. Reproduction takes place by swarmspores, which have 1 or 2 cilia, and sexual reproduction in some by gamete-conjugation. The principal genera are: _Chlorococcum_, _Chlorochytrium_, _Chlorocystis_, _Scotinosphæra_, _Endosphæra_, _Phyllobium_, _Characium_, _Ophiocytium_, _Sciadium_. [Illustration: FIG. 49.--_Characium strictum._ _A_ The cell-contents have divided into many swarmspores. _B_ Swarmspores escaping.] Order 6. =Hydrodictyaceæ.= The individuals are unicellular but several unite after the zoospore-stage into definitely formed families (cœnobia). Ordinary vegetative division is wanting, but asexual reproduction takes place by zoospores (or by motionless cells without cilia), which unite and form a family similar to the mother-family, inside the mother-cell, or in a mucilaginous envelope. Where sexual reproduction is found it takes place by gamete-conjugation. The principal genera are: _Pediastrum_ (Fig. 50), _Cœlastrum_, _Hydrodictyon_ (Fig. 51). [Illustration: FIG. 5O.--_Pediastrum asperum._]