The paper by Apstein (1911) is the only previous work known to me which is concerned with parasites of marine pelagic Crustacea in general, chiefly Copepoda, and especially of Calanus finmarchicus. It is of the nature of a preliminary note, since his work in the neighbourhood of Kiel was suddenly interrupted in its early stages; so that, while some of his “parasites” are easily identifiable, the scanty notes on others are difficult of interpretation and have given much trouble to subsequent writers. After making observations on the parasites of Calanus in the Clyde Sea Area over a period of several years, I feel in a position to offer the following opinions on Apstein's note together with an account of very early stages in the development of some larval Platyhelminthes.