The Family may be divided into 2 sub-families:
1. Homocysteæ (heterocysts are wanting): Chroococcaceæ, Lyngbyaceæ and Chamœsiphonaceæ.
2. Heterocysteæ (heterocysts present): Nostocaceæ, Rivulariaceæ, Scytonemaceæ and Sirosiphoniaceæ.
Order 1. Chroococcaceæ. The individuals are 1—many-celled, but all the cells are uniform, united to form plates or irregular masses, often surrounded by a mucilaginous cell-wall, but never forming cell-chains. Multiplication by division and sometimes by resting akinetes, but reproductive akinetes are wanting. Chroococcus, Aphanocapsa, Glœocapsa (Fig. [20]), Cœlosphærium, Merismopedium, Glaucocystis, Oncobyrsa, Polycystis, Gomphosphæria.
Fig. 20.—Glœocapsa atrata: A, B, C, D, E various stages of development.
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.