In view of the magnitude of the task which my enthusiasm, perhaps unwarrantable, has led me to undertake, I know full well how considerately my foreign friends and correspondents will deal with the errors of omission and commission that they will certainly detect in these pages. If there be any educated persons at home who still affect to despise the revelations of helminthology, I can assure them that their prejudices are misplaced. The study of the structure and economy of a humble parasite brings to the investigator no slight insight into the workings of nature. If these workings cannot at all times be pronounced to be “good and beautiful,” they must at least be characterised as “true.” The knowledge of the true--especially if that knowledge by its practical applications be calculated to confer substantial benefits upon man and his inferior fellow-creatures--ought to be held in high esteem; but, apart from this purely utilitarian view, there remains for the investigator the delight occasioned by the in-rush of new scientific ideas. The average mind, being either essentially commercial or ridiculously sentimental, as the case may be, is totally incapable of comprehending the motive power that animates and guides the votary of science. The late Professor Faraday, a man wholly untinged by the ambitions of wealth and power, once remarked to me that there were no people so difficult to instruct as those who were ignorant of their own ignorance. It is just these very persons who, when placed in high positions of social, political, or professional trust, most powerfully contribute to check a nation’s progress. There are too few genuine workers at science in this country. As one of the rank and file, I claim only to have honestly contributed my mite. I should like to see a small army of helminthologists rise up and lay siege to the fortresses at present securely held by thousands of death-dealing parasites. T. S. C. 74, Portsdown Road, London _May, 1879_. SYNOPSIS OF CONTENTS. PAGE GENERAL INTRODUCTION 1 BOOK I. PARASITES OF MAN. SECTION I.--TREMATODA (FLUKES) 14 " II.--CESTODA (TAPEWORMS) 56 " III.--NEMATODA (ROUNDWORMS AND THREADWORMS) 149 " IV.-- PART I.--Acanthocephala (Thornheaded Worms) 256 " II.--Suctoria (Leeches) 257 " III.--Arachnida (Parasitic forms of) 259 " IV.--Crustacea (alleged Parasitic forms of) 268 " V.--Insecta (Parasitic forms of) 269 " VI.--Protozoa (Parasitic forms of) 276