During the course of the last few years our knowledge of parasitic worms in all their various relations has advanced and increased to a very considerable and satisfactory extent. From the purely zoological point of view the advances have been of the highest order, many important discoveries having been made, not only in connection with the morphology but also, and particularly, in connection with the development and bionomics of these worms. At the same time there has been a steadily increasing improvement in our ideas of their classification and relationships which has resulted in a much wider and more accurate conception of their zoological nature. While these rapidly accumulating discoveries have greatly extended the knowledge at our disposal, they have, in addition, rendered such facts easier of study and of use, with the result that the economic value of the subject as a whole has been very materially enhanced. Helminthology shares with Protozoology and Entomology the, in some respects, unenviable distinction of being of intrinsic importance not only to medicine, but also to veterinary and agricultural science. It has a smaller but no less definite economic importance in relation to fisheries. Apart from their purely zoological interest it is particularly as agents of disease that the parasitic worms are of importance, but there are a number of other matters connected with habits and distribution, on which a study of parasites may throw considerable light.