It is well known that certain marine polychaetes of the European and American coasts are often found in the brackish waters of estuaries or even in the fresh water of the lower part of rivers but apparently without being able to breed in these locations. There are, however, certain forms so extremely adaptible that they seem to live and breed equally well in marine brackish and fresh water, such being notably the South American Namanereis ouanaryensis (Gravier) and Namanereis quadraticeps (Gay). Other species of this same genus Namanereis (Lycastis of most authors) are found only in fresh water, and representatives of several other genera of Polychaeta, mostly monotypic, are likewise restricted to fresh water.