Among collections of meiobenthos from some sub-littoral muds off the coast of Scotland were eighty-five species of Copepoda. Twelve of these were found to be new species of Harpacticoida. In addition, the hitherto unknown males of four species were discovered.

The introduction contains an appreciation of the work of James Ritchie (1882–1958) on hydroids. His collections in the Royal Scottish Museum consisting of 266 species have been re-examined and listed with up-to-date synonymy, this also serving as a catalogue of all the identified hydroid material in this Museum.

Among the important collections are those made by W. S. Bruce in the Scotia and E. H. Shackleton's Antarctic Expeditions; there is also much valuable secondary material, especially in the form of micro-preparations, from collections reported on by Ritchie, notably those of the Australian Thetis Expedition, the Scottish Loch collections of Sir John Murray, the Indian Museum and collections by Cyril Crossland, James Simpson and Rudmose Brown and others.

Ritchie described many new species and varieties and all the type material has been noted, lectotypes being designated where necessary.

Hydroids from the Goldseeker collections of the Fishery Board for Scotland were not reported on by Ritchie although he identified a considerable number of samples from Scottish waters—these have been included in this survey. These new records are of considerable value for compiling distribution lists. Apart from Ritchie's own work little has been done on hydroids from waters around the north of Scotland since the days of Hincks, Allman and Norman. The bibliography contains a full list of James Ritchie's hydroid papers.