The studies of which the results are here set forth were carried on at the Granton Marine Laboratory of the Scottish Meteorological Society, in the years 1886 and 1887. Our memoir is by no means a monograph, although our original aim was to investigate every species taxonomically, anatomically, and embryologically. Much further study would have been necessary to carry out this aim at all completely, but in August 1887, both of us, for different reasons, had to abandon our work at Granton and leave Scotland. We have thought it better to publish the notes and drawings we had made, because they will probably be of service to British naturalists interested in the Polychæta, no extensive work on these forms having appeared in English since Johnston's Catalogue of Non-Parasitical Worms in the British Museum, which was published 1865, and which is now a very inadequate guide to the study of the subject. The greater part of the work of collecting, and much the larger part of the drawings, were done by Mr Ramage. A discussion of some anatomical points has been published separately by Mr Cunningham (“Some Points in the Anatomy of Polychæta,” Quart. Jour. Micr. Sci., 1887).