Order 4. =Mycoideaceæ.= The thallus is discoid, consisting of one or more cell-layers, and is always attached. Asexual reproduction by zoospores with 2 or 4 cilia. Sexual reproduction in some species by the conjugation of gametes with 2 cilia. This order forms the connecting link between _Chætophoraceæ_ and _Coleochætaceæ_. The species occur in fresh water (_Chætopeltis_) as well as in salt (_Pringsheimia_), on the carapace of tortoises (_Dermatophyton_ = _Epiclemmydia_), or endophytic between the cuticle and the epidermal cells of the leaves of tropical plants, destroying the leaf-tissue (_Mycoidea_). Order 5. =Cylindrocapsaceæ.= The thallus consists of a simple (rarely, in parts, formed of many rows) unbranched filament, attached in the young condition, which has short cells with a single nucleus, and is enveloped in a thick envelope with a laminated structure. Asexual reproduction by zoospores with 2 cilia, which are formed 1, 2, or 4 in each vegetative cell. The antheridia are produced by a single cell, or a group of cells, in a filament, dividing several times without increasing in size. Two egg-shaped spermatozoids, each with 2 cilia (Fig. 53 _D_), are formed in each antheridium, and escape through an aperture in the side; in the first stages they are enclosed in a bladder-like membrane (Fig. 53 _B_, _C_). Other cells of the filament swell out and form oogonia (Fig. 53 _A_), which resemble those of _Œdogonium_. After fertilisation, the oospore surrounds itself with a thick wall, and assumes a reddish colour. The germination is unknown. The unfertilised oospheres remain green, divide often into 2–4 daughter-cells, and grow into new filaments. [Illustration: FIG. 53.--_Cylindrocopsa involuta. A_ Oogonium with oosphere (_o_) surrounded by spermatozoids (_s_). _B_ Two antheridia, each with two spermatozoids. _C_ Spermatozoids surrounded by their bladder-like membrane. _D_ Free spermatozoid.] This order, which only includes one genus, _Cylindrocapsa_, forms the connecting link between _Ulothricaceæ_ and _Œdogoniaceæ_. The few species (4) occur only in fresh water.