1. Confervaceæ—Frond thread-like, composed of cylindrical cells placed end to end. Spores very minute, formed within the cells.
2. Ulvaceæ—Frond grass-green or purple, flat or tubular. Spores minute, ciliated, formed in the cells of the frond.
3. Siphonaceæ—Frond a single, thread-like, branching cell, or a spongy mass of many such cells.
B. Rhodospermeæ—Red-spored weeds. Spores in globular conceptacles. Tetraspores (four-clustered spores) in globular or cylindrical cells. Frond red, reddish brown, or purple.
4. Ceramiaceæ—Frond thread-like, jointed, one-siphoned, and more or less covered with a layer of cortical cells. Spores grouped in transparent, membranous sacs, sometimes surrounded by a whorl of short branchlets.
5. Spyridiaceæ—Frond thread-like, jointed, one-siphoned, more or less covered with small cells. Spores formed in the upper cells of branched, jointed, radiating threads, enclosed in a cellular membrane in external conceptacles.
6. Cryptonemiaceæ—Frond more or less cartilaginous, composed of numerous jointed threads compacted by gelatine. Spores grouped without order in internal cells or in external conceptacles.
7. Rhodymeniaceæ—Frond inarticulate, membranaceous, composed of polygonal cells, the surface cells forming a continuous layer. Spores in beaded threads in external conceptacles.
8. Wrangeliaceæ—Frond inarticulate, thread-like, traversed by a jointed tubular axis. Spores formed in the terminal cells of clustered, branching, naked threads.
9. Helminthocladiæ—Frond cylindrical, gelatinous, composed of filaments imbedded in gelatine. Spores formed on branching, radiating threads that are enclosed in the frond without conceptacles.