The sexual reproductive organs are formed on short lateral branches, and are separated from the vegetative cell (Fig. 58 _A_) by cell-walls. Numerous spermatozoids, each with two cilia, are developed in the coiled antheridium (_A_, _b_). The oogonium is a thick, egg-shaped, often oblique cell, with its protoplasm rounded into an oosphere, which has a hyaline “receptive-spot” (_A_, _a_) immediately beneath the aperture formed in the wall of the oogonium. A slimy mass, which serves to receive the spermatozoids, is formed in some species in this aperture. The spermatozoids when liberated swim towards and enter the oosphere, which then immediately surrounds itself with a thick cell-wall. The mature oospore (_B_) contains a large quantity of oil. At germination the outer cell-wall bursts and a new plant is formed. There is only one genus, _Vaucheria_, with species living in salt as well as in fresh water and on damp soil. [Illustration: FIG. 58.--_Vaucheria sessilis_. _A_ Fertilisation; _b_ the antheridia; _a_ the oogonia; _a_ the receptive spot. _B_ Oospore.] Order 5. =Phyllosiphonaceæ= are parasites in the leaves and stalks of Flowering-plants. Order 6. =Caulerpaceæ.= The thallus has distinct differentiation into root, stem and leaf-like members (Fig. 59); it is unicellular. Within the cell, strong, branched threads of cellulose extend from one side to the other serving as stays to support the thallus. Reproduction takes place by detached portions of the thallus; no other modes of reproduction are known. This order may most approximately be classed with the _Bryopsidaceæ_. The genus _Caulerpa_ consists of more than seventy species which inhabit the tropical seas. Order 7. =Codiaceæ.= The thallus has various forms, but without distinct differentiation in stem- or leaf-structures, sometimes (_e.g._ _Halimeda_) it is very much incrusted with lime. In the early stages it is unicellular (later, often multicellular), very much branched, with the branches, at any rate partly, so united or grown in amongst one another (Fig. 60) that an apparently parenchymatous cellular body is formed. Akinetes or aplanospores are wanting; zoospores (or gametes?) may be developed in some species, however, in special swollen sporangia. Fertilisation similar to that in _Bryopsis_ occurs perhaps in _Codium_. They are all salt water forms.