[Illustration: FIG. 20.--_Glœocapsa atrata_: _A_, _B_, _C_, _D_, _E_ various stages of development.] [Illustration: FIG. 21.--_Oscillaria_; =a= terminal, =b= central portion of a filament.] Order 2. =Lyngbyaceæ (Oscillariaceæ).= The cells are discoid (Fig. 21), united to straight or spirally twisted, free filaments, which are unbranched, or with spurious branching. The ends of the cell-chains are similar. Heterocysts absent. Reproduction by synakinetes, resting akinetes are wanting. _Oscillaria_ (Fig. 21), _Spirulina_, _Lyngbya_, _Microcoleus_, _Symploca_, _Plectonema_. Order 3. =Chamæsiphonaceæ.= The individuals are 1--many-celled, attached, unbranched filaments with differentiation into apex and base, without heterocysts. Multiplication by reproductive akinetes; resting akinetes are wanting. _Dermocarpa_, _Clastidium_, _Chamæsiphon_, _Godlewskia_, _Phragmonema_. Order 4. =Nostocaceæ.= The individuals are formed of multicellular, unbranched filaments, without differentiation into apex and base; heterocysts present. Reproduction by synakinetes and resting akinetes. [Illustration: FIG. 22.--_Nostoc verrucosum. A_ The plant in its natural size; an irregularly folded jelly-like mass. _B_ One of the cell-chains enlarged, with its heterocysts (_h_), embedded in its mucilaginous sheath.] Some genera are not mucilaginous, _e.g. Cylindrospermum_ (Fig. 19). The cell-chains in others, _e.g. Nostoc_, wind in between one another and are embedded in large structureless jelly-like masses, which may attain the size of a plum or even larger (Fig. 22); sometimes they are found floating in the water, sometimes attached to other bodies. Other genera as follows: _Aphanizomenon_ and _Anabæna_ (in lakes and smaller pieces of water); _Nodularia_ is partly pelagic. Some occur in the intercellular spaces of higher plants, thus _Nostoc_-forms are found in _Anthoceros_, _Blasia_, _Sphagnum_, _Lemna_, and in the roots of _Cycas_ and _Gunnera_; _Anabæna_ in _Azolla_. Order 5. =Rivulariaceæ.= The individuals are multicellular filaments, with differentiation into apex and base; spurious branching, and a heterocyst at the base of each filament, reproduction by synakinetes and resting akinetes, rarely by simple reproductive akinetes. _Rivularia_, _Glœotrichia_, _Isactis_, _Calothrix_. Order 6. =Scytonemaceæ.= The individuals are formed of multicellular filaments with no longitudinal division; differentiation into apex and base very slight or altogether absent; branching spurious; heterocysts present. Reproduction by synakinetes, rarely by resting akinetes and ordinary reproductive akinetes. _Tolypothrix_, _Scytonema_, _Hassalia_, _Microchæte_.