The cilia of _Volvox_, of which there are two to each of the component cells, are difficult to detect; they are best seen when the organism is dried without a cover, or after moistening them with a little solution of iodine, which dyes them brown. _Synúra volvox_ (Pl. VI. fig. 13) is a still more minute member of this family, and is often found rolling along among _Confervæ_. The greenish zoospore-like bodies of this Alga have one cilium only, and arise from a common centre by a narrowing of the base (fig. 14). In _Gónium pectorále_ (fig. 11 _a_) the green bodies, which are sixteen in number, and furnished each with two cilia, are grouped into a flat square plate; and in the very minute _Gonium tranquil´lum_ (fig. 11 _c_) these bodies are also sixteen in number, and arranged in a tabular form, but are without cilia. SIPHONA´CEÆ.--The structure of this family may be illustrated by the genus _Vauchéria_, of which two or three species are common on damp ground or in freshwater pools, forming a green layer. At first sight, the filaments of which the little plants consist appear like those of a stout _Conferva_; but on close examination they are found to be branched, and not jointed, consisting of a single cell from end to end (Pl. VI. fig. 26). The reproduction is effected by the agency of two kinds of organs, antheridia and capsules (sporangia), situated near each other (fig. 26 _a_) on the walls of the filaments, of which they are protrusions or outgrowths--their cavities being separated from that of the filament by a partition or septum. The antheridia produce spermatozoa, and the sporangia each a spore, the one fertilizing the other in the ordinary manner. In addition to this method of fructification, zoospores are also produced--the ends of the filaments becoming swollen, the contents cut off by a septum, and forming single large zoospores covered with cilia, the further development of which resembles that occurring in the Confervaceæ. OSCILLATORIA´CEÆ.--The members of this family are commonly found in stagnant water or on shaded damp ground, especially in the cold seasons of the year, forming green strata or masses. PLATE VI. [PAGE 84.] FRESHWATER ALGÆ. Fig. 1. _Oscillatoria autumnalis._ 2. _Oscillatoria nigra._ 3. _Coscinodiscus radiatus._ 4. _Nostoc minutissimum._ 5. _Actinocyclus undulatus._ 6. _Bacterium._ 7. _Rhabdonema arcuatum._ 8. _Rhabdonema arcuatum_, prepared frustules. 9. _Melosira nummuloides._ 10. _Melosira nummuloides_, prepared frustules. 11. _a_, _b_, _Gonium pectorale_: 11 _c_. _Gonium tranquillum._ 12. _Spirulina oscillarioides._ 13. _Synura volvox._ 14. _Synura volvox._ 15. _Gyrosigma attenuatum_, front view. 16. _Gyrosigma attenuatum_, side view; 16 _a_, portion of a valve. 17. _Gyrosigma angulatum_; 17 _a_, portion of a valve. 18. _Volvox globator._ 19. _Glœocapsa._ 20. _Chara vulgaris_, globule. 21. _Chara vulgaris_, portion of filament.