The origin of the present investigation was an attempt to gain information as to the character of the coccospheres and rhabdospheres by employing the pumping method, first used by Sir John Murray at sea in place of tow-netting. It was considered that the difficulty and great expense of obtaining tow-nettings remote from land, involving the charter of a vessel, might be overcome by inducing the commander of a steamship to pump water through fine silk nets and to prepare and preserve in a suitable manner the proceeds obtained in this way. We have described in Nature’ (April 1, 1897) the success of this method in the hands of Captain W. Haultain Milner, then of B.M.S. “ Para,” one of the Boyal Mail Steam Packet Company’s ships. He obtained for our study not only specimens of coccospheres, but of both kinds of rhabdospheres, of Pyrocystis noctiluca and other organisms considered by many to be of uncertain position. While he was engaged on the second voyage during which he carried out these operations, we were enabled, by a grant from the Government Grant Fund and by special leave of absence from the Trustees of the British Museum, to prepare to accompany him on his third voyage, in order to make observations on living material at sea. We made this voyage from Southampton to Barbados, Hayti, Jamaica and Colon (Panama) and back. The unwearying services and numerous valuable suggestions of Captain Milner, Mr. Jolliffe, the chief officer of the “Para,” and Mr. Hindmarch, the chief engineer, while we were aboard, contributed greatly to such success as we may have attained. Our knowledge of the phyto-plankton of the high seas has been derived almost exclusively from the observations and collections made by zoologists, who have advanced their part of the study of plankton with far greater zeal than has been shown by botanists. The “Challenger” Expedition left many problems unsettled and the Hensen Expedition, accompanied by a botanist (Dr. Schütt) skilled in the minute examination of such organisms, though it accomplished great things, still left a legacy of doubts on many points, and in fact proclaimed more loudly than ever the richness of this field of investigation.