Order 1. =Ectocarpaceæ.= The thallus consists of single or branched filaments with intercalary growth, extending vertically from a horizontal, branched filament or a disc, but sometimes it is reduced to this basal portion only. Zoosporangia and gametangia (for fertilisation see Fig. 66) are either outgrowths or arise by the transformation of one or several of the ordinary cells. The most common genera are: _Ectocarpus_ and _Pylaiella_. Order 2. =Choristocarpaceæ.= _Choristocarpus_, _Discosporangium_. Order 3. =Sphacelariaceæ.= The thallus consists of small, parenchymatous, more or less ramified shoots, presenting a feather-like appearance. In the shoots, which grow by means of an apical cell (Fig. 68 _S_), a cortical layer, surrounding a row of central cells, is present. Sporangia and gametangia are outgrowths from the main stem or its branches. _Sphacelaria_, _Chætopteris_ are common forms. [Illustration: FIG. 68.--Apex of the thallus of _Chætopteris plumosa_. _S_ Apical cell.] Order 4. =Encoeliaceæ.= _Punctaria_, _Asperococcus_, _Phyllitis fascia_. Order 5. =Striariaceæ.= _Striaria_, _Phlœospora_. Order 6. =Dictyosiphonaceæ.= _Dictyosiphon._ Order 7. =Desmarestiaceæ.= _Desmarestia aculeata_ is common. Order 8. =Myriotrichiaceæ.= _Myriotrichia._ Order 9. =Elachistaceæ.= _Elachista fucicola_ is a common epiphyte on species of _Fucus_. Order 10. =Chordariaceæ.= The shoot-systems are often surrounded by mucilage. _Chordaria_; _Leathesia difformis_ occurs as rounded, brown-green masses of the size of a nut, generally attached to other Seaweeds. Order 11. =Stilophoraceæ.= _Stilophora rhizodes_ is common. Order 12. =Spermatochnaceæ.= _Spermatochnus paradoxus_ is common. Order 13. =Sporochnaceæ.= _Sporochnus._ Order 14. =Ralfsiaceæ.= _Ralfsia verrucosa_ is common as a red-brown incrustation on stones and rocks at the water’s edge. Order 15. =Lithodermataceæ.= Some species of the genus _Lithoderma_ occur in fresh water. [Illustration: FIG. 69.--_Laminaria digitata_ (much reduced in size).]