Order 6. =Œdogoniaceæ.= The thallus consists of branched (_Bulbochæte_) or unbranched (_Œdogonium_) filaments, attached in the early stages. The cells may be longer or shorter, and have one nucleus. Asexual reproduction by zoospores, which have a chaplet of cilia round the base of the colourless end (Fig. 6 _a_). Sexual reproduction takes place by oogamous fertilisation. On the germination of the oospore, 4 zoospores are formed (Fig. 54 _F_). They occur only in fresh or slightly brackish water. The division of the cells takes place in quite a peculiar and unusual manner. At the upper end of the cell which is about to divide, a ring-shaped thickening of soft cellulose is formed transversely round the wall; the cell-nucleus of the mother-cell and the protoplasm then divide by a transverse wall into two portions of similar size, and the cell-wall bursts transversely along the central line of the thickened ring. The cell-wall thus divides into two parts--the upper one short, the “cap,” and the lower one much longer, the “sheath.” The portions of the original cell-wall now separate from each other, the cellulose ring extending, and supplying an additional length of cell-wall between them. The cap and sheath will project a little in front of the piece thus inserted. The dividing wall between the two new cells is formed near to the uppermost edge of the sheath, and gradually becomes thicker and firmer. The inserted piece of wall forms the larger part of the wall of the upper cell: the remainder is formed by the cap. This mode of division is repeated exactly in the same way, and new caps are formed close below the first one, one for every division. [Illustration: FIG. 54.--_A Œdogonium ciliatum. A_ Female plant with three oogonia (_og_) and dwarf-males (_m_). _B_ An oogonium with spermatozoid (_z_) seen entering the oosphere (_o_) having passed through an aperture near the summit of the oogonium; _m_ dwarf-male. _C_ Ripe oospore. _D Œdogonium gemelliparum. F_ Portion of a male filament from which spermatozoids (_z_) are emerging. _E_ Portion of filament of _Bulbochæte_; the upper oogonium still encloses the oospore, in the central one the oospore is escaping while the lower one is empty. _F_ Four zoospores developed from an oospore. _G_ Zoospore germinating.]