Order 8. =Cladophoraceæ.= This order is probably derived from the Ulothricaceæ. The thallus consists of a single, unbranched or branched filament, generally with an apical cell. The cells have each 2 or more nuclei. Asexual reproduction by zoospores with 2 or 4 cilia, and by akinetes. Conjugation of gametes with 2 cilia is found in some genera. They occur in salt as well as in fresh water. The principal genera are: _Urospora_, _Chætomorpha_, _Rhizoclonium_, _Cladophora_; of the last named genus the species _C. lanosa_ and _C. rupestris_ are common in salt water; _C. fracta_ and _C. glomerata_ in fresh water. Order 9. =Gomontiaceæ.= _Gomontia polyrrhiza_, the only species hitherto known, is found on old calcareous shells of certain salt water Molluscs. Order 10. =Sphæropleaceæ.= The thallus consists of free, unbranched filaments, with very elongated multinuclear cells. The vegetative cells form no zoospores. Sexual reproduction by oogamous fertilisation (see page 13, Fig. 10 _B_). The oospore has a thick wall (Fig. 10 _D_) studded with warts, and assumes a colour resembling red lead. It germinates only in the following spring, and produces 1–8 zoospores, each with 2 cilia (Fig. 10 _E_), which grow into new filaments. Only one species, _Sphæroplea annulina_, is known. Family 3. Siphoneæ. The thallus has apical growth, and in the vegetative condition consists generally of one single (in the Valoniaceæ most frequently of more) multinuclear cell, which may be much branched, and whose separate parts in the higher forms (_e.g. Bryopsis_, Fig. 57; _Caulerpa_, Fig. 59, etc.) may be differentiated to perform the various physiological functions (as root, stem and leaf). Vegetative multiplication by detached portions of the thallus (gemmæ); asexual reproduction by zoospores, akinetes, or aplanospores. Sexual reproduction by gamete-conjugation, rarely by oogamous fertilisation. The zygote or oospore germinates as a rule without any resting-stage. [Illustration: FIG. 56.--_Botrydium granulatum_: _a_ an entire plant forming swarmspores; _b_ swamspores; _c_ an individual with gametangia; _d_, gamete; _e_, _f_, _g_ conjugation; _h_ zygote seen from above; _i_ the same in a lateral view.] Most of the Siphoneæ occur in salt water or on damp soil. Many (_e.g._ _Dasycladaceæ_) are very much incrusted with lime, and occur, in the fossilized condition, in the deposits from the Cretaceous period to the present time. The Siphoneæ are connected by their lowest forms (_Botrydiaceæ_ or _Valonia_) with the Protococcaceæ, but show also, through the Valoniaceæ, points of relationship to the _Cladophoraceæ_.