When the work was considered with a view to a new edition, it was found that a vast amount of new matter had to be incorporated, numerous alterations essential for bringing it up to date were necessitated, and many omissions were inevitable. The result is that parts of the book have been rewritten, and, apart from early historical references, the work of Braun has disappeared. This is more particularly the case with the Protozoa section of the present work. The numerous additions, due to the great output of scientific literature and other delays in publication, have led to the book being somewhat less homogeneous than we desired, and have necessitated the use of appendices to allow of the presentation of new facts only recently ascertained. Many new illustrations have been added or substituted for older, less detailed ones. Some of these new figures were drawn specially for this book. The first section, on the Protozoa, has been written by Dr. Fantham, there being little of the original text left except parts of the historical portions, and thus the section on Protozoa must be considered as new. The second section, on Worms (except the Acanthocephala, Gordiidæ and Hirudinea), has been remodelled by Professor Stephens to such an extent that this, too, must not be looked upon as a translation of Braun’s book. With regard to the Arthropoda, much remains as in the last English edition, but some new matter added by Braun in his fourth German edition is included, and much new matter by Mr. Theobald has been incorporated. As regards the Appendix by Dr. Seifert, the first section has been remodelled, but the sections on the Helminthes and the Arthropoda are practically translations of the original. The authors desire to express their thanks to Miss A. Porter, D.Sc., J. P. Sharples, Esq., B.A., M.R.C.S., and H. F. Carter, Esq., F.E.S., for valuable help. They also wish to thank the authors, editors, and publishers of several manuals and journals for their courtesy in allowing the reproduction of certain of their illustrations. In this connection mention must be made more particularly of Professor Castellani, Dr. Chalmers, Professor Doflein, Dr. Leiper, the late Professor Minchin, Professor Nuttall, Dr. Wenyon, Mr. Edw. Arnold, Messrs. Baillière, Tindall and Cox, Messrs. Black, Messrs. Cassell, Dr. Gustav Fischer, Messrs. Heinemann, the Cambridge University Press, the Editors of the _Annals of Tropical Medicine and Parasitology_, the Editors of the _Journal of Experimental Medicine_, and the Editor of the _Tropical Diseases Bulletin_. H. B. F. J. W. W. S. F. V. T. _December, 1915._ CONTENTS. PAGE PREFACE iii ERRATA xxxii