LONDON: J. & A. CHURCHILL, NEW BURLINGTON STREET. 1879. PREFACE My introductory treatise on the Entozoa having long been out of print, it occurred to me that instead of attempting another edition it would be better to write an entirely new work, employing only such fragmentary portions of the old treatise as would harmonise with the far wider design I have now in view. Whilst, therefore, I have freely utilised a selection of the illustrations given in the elementary volume, comparatively few of its pages have been incorporated in the present work. Dealing with parasites and parasitism after a manner not hitherto attempted I have purposely omitted minute anatomical descriptions, and, with rare exceptions, I have avoided the introduction of clinical details. While bringing to a focus the records of, and principal references to, a widely scattered, intricate, and voluminous literature, it has been my chief endeavour to supply abundance of original matter of a kind that cannot be found in the columns of any existing treatise. Whether I have succeeded or not the experienced helminthologist alone can judge. He, at all events, will perceive that the summary, though compressed within the space of a moderate-sized octavo, can only have resulted from sustained effort. This treatise is not professional, that is to say, it does not concern itself with therapeutics or the curative treatment of parasitic affections; yet it introduces and helps to solve many questions relating to epidemics, endemics, and epizoötics due to parasites. The medical man who only looks at the phenomena of parasitism as displayed within the human territory must of necessity acquire a cramped, narrow, and distorted conception of the rôle played by parasites in the production of disease. Let it be freely granted that to the practising physician, as such, it matters little how many beasts, birds, reptiles, or fishes perish annually from parasitic affections; yet, when it is demonstrable that a large proportion of the strictly human entozoa require a change of hosts--or, in other words, need to pass through the bodies of the lower animals--then it is evident that some acquaintance on his part with the entozoa infesting animals becomes a practical necessity. Knowledge of the kind here offered will often materially aid him in recommending prophylactic measures. Moreover, the study of comparative pathology, almost ignored in England, conveys with it other lessons of high value in relation to the healing art. The great mind of John Hunter comprehended all this long ago, as any student of the beautiful preparations contained in the museum of the Royal College of Surgeons may readily convince himself; and this is all the more noteworthy, since the subject concerns the physician rather than the surgeon.