[Illustration: FIG. 51.--_Hydrodictyou reticulatum. A_ A cell where the zoospores are on the point of arranging themselves to form a net. _B_ A cell with gametes swarming out.] The cœnobium of _Hydrodictyon reticulatum_ (Water-net) is formed of a large number of cells which are cylindrical, and attached to one another by the ends (Fig. 51). The asexual reproduction takes place by zoospores, which are formed in large numbers (7,000–20,000) in each mother-cell, within which they move about for a time, and then come to rest and arrange themselves into a new net (Fig. 51 _A_) which is set free by the dissolution of the wall of the mother-cell, grows, and becomes a new cœnobium. The sexual reproduction takes place by gamete-conjugation. The gametes are formed in the same manner as the zoospores, but in larger numbers (30,000–100,000), and swarm out of the mother-cell (Fig. 51 _B_). The zygote forms, on germination, 2–5 large zoospores, each with one or two cilia, these generally swarm about for a time, and after a period of rest become irregular thorny bodies (polyhedra); their contents again divide into zoospores, the thorny external coating of the polyhedra is cast off, and the zoospores, surrounded by the dilated internal coating, unite to form a small family, which produces several others in the manner described. Family 2. =Confervoideæ.= The individuals are always multicellular, the cells firmly bound together and united into unbranched or branched filaments, expansions, or masses of cells which grow by intercallary divisions or have apical growth. In the first seven orders the cells are uninuclear, but the cells of the remaining three orders contain several nuclei. Asexual reproduction by zoospores, akinetes or aplanospores. Sexual reproduction by isogamous or oogamous fertilisation. The Confervoideæ, through the Ulvaceæ, are connected with the Tetrasporaceæ, and from the _Coleochætaceæ_, which is the most highly developed order, there are the best reasons for supposing that the Mosses have taken their origin. The _Cladophoraceæ_ show the nearest approach to the _Siphoneæ_. Order 1. =Ulvaceæ.= The thallus consists of one or two layers of parenchymatous cells, connected together to form either a flat membrane (_Monostroma_, _Ulva_) or a hollow tube (_Enteromorpha_), and may be either simple, lobed, or branched. Reproduction takes place by detached portions of the thallus; or asexually by zoospores or akinetes. Gamete-conjugation is known to take place in some members of this order, the zygote germinating without any resting-stage. The majority are found in salt or brackish water.