Further on the general advantages to be derived from the study of parasites I cannot here dilate, and it becomes the less necessary that I should do so, since I have entered upon the subject very fully elsewhere. The character of the present work, moreover, imposes brevity. If the plan which I now propose to follow should not be deemed altogether satisfactory from the purely zoological standpoint, it will nevertheless have the advantage of simplicity and novelty; and knowing full well the difficulties that must surround any attempt to give a perfect classification of the entozoa, considered as a natural group, I feel sure that my helminthological friends will credit me with exercising a wise discretion in selecting the simplest available method of arrangement. My plan, therefore, is to devote separate sections of this work to the parasites of the different classes of vertebrated animals, including man, treating of the various species in regular succession. This arrangement is merely one of convenience and has no reference whatever to conceptions of zoological equivalency as variously interpreted and maintained by authors and investigators. The parasitic groups will be taken up in the following order, quite irrespective of their relative importance, and also without any attempt to treat each group with equal fulness. In the matter of recent literature only will the present record and summary make any approach toward completeness, my hope being to render this treatise indispensable and trustworthy as a ready means of reference. I. FLUKES. TREMATODA.--This group embraces several families of parenchymatous worms. The various species exhibit one or more suckers, which the older naturalists regarded as so many mouths or _perforations_. Hence the ordinal title. The term fluke is of Saxon origin, meaning anything flat. Thus, it has been applied to sole-fish or flounders, to the flattened halves of the tail of cetaceans, to the blades of anchors, and so forth. Although the common liver fluke is flat, many species of the order are round, biconvex, or even filiform organisms. I recognise six families:--_Monostomidæ_, _Distomidæ_, _Amphistomidæ_, _Tristomidæ_, _Polystomidæ_, and _Gyrodactylidæ_. Most of the species are entozoal; but many adhere to the surface of the body of piscine hosts.