In so wide a field it is almost necessary to exercise some favour itism in the choice of objects, and greater stress has been laid in this work upon the parasitic forms, both on account of the many interesting biological problems which they present, and also because they come into closer relationship with the practical needs of human life than the non-parasitic species. The author wishes, however, to point out clearly that he is not a medical man, but one who approaches the study of the parasitic Protozoa solely from the standpoint of a naturalist who is more concerned, so to speak, with the interests of the parasite than with those of the host. Conse quently, purely medical problems — such as, for example, the symptoms and treatment of diseases caused by trypanosomes and other Protozoa — are not dealt with in this book, since the author deems it no part of his task to attempt to instruct medical men concerning matters with which they are better acquainted by their training and experience than himself. The needs of medical men have, however, been specially kept in view, and the author hopes that the book will succeed in supplying them with useful informa tion, at least from a general zoological or biological standpoint.