Division I. ALCYONELLEA.
The zoœcia arise directly from one another in a fleshy or gelatinous mass. The polypide has no gizzard. The species are essentially marine, but a few are found in brackish water in estuaries.
Division II. STOLONIFERA.
The zoœcia arise from expansions in a delicate creeping rhizome or root-like structure, the order in which they are connected together being more or less irregular. As a rule (perhaps always) there is no gizzard. The species are marine.
The zoœcia grow directly from a tubular stem which is usually free and vertical, their arrangement being alternate, spiral or irregular. There is a stout gizzard which bears internal chitinous projections and is tightly compressed when the polypide is retracted. The species are essentially marine, but a few are found in brackish water.
Division IV. PALUDICELLINA, nov.
The zoœcia are arranged in a regular cruciform manner and arise either directly one from another or with the intervention of tubular processes. If the polypide has a gizzard it does not bear internal chitinous projections. Most of the species are confined to fresh water, but a few are found in brackish water or even in the sea.
Although all true freshwater Ctenostomes belong to the fourth of these divisions, species of a genus (Bowerbankia) included in the third are so frequently found in brackish water and in association with one belonging to the fourth, and are so easily confounded with the latter, that I think it necessary to include a brief description of the said genus and of the form that represents it in ponds of brackish water in India.