To the naturalist the second half of this book addresses itself in a very direct manner. When engaged in his dissections, an appeal to its pages will often enable him to decide at once as to the species of parasite accidentally encountered, and if a full diagnosis be demanded it will guide him to better sources of information. Many hundreds of correspondents, not having ready access to the systematic writings of Rudolphi, Diesing, and Dujardin, have requested me to identify their “finds.” I have rarely or never failed to comply with their requests; but it is hoped that the present work may prove of ready service to subsequent inquirers, and thus place a reasonable limit upon the number of future applicants. Since the manuscript of this work was completed I have received Dr von Linstow’s _Compendium der Helminthologie_, which, for the purposes held in view by the author, leaves little to be desired. Expressly to meet the requirements of the Sanitarian I have dwelt upon the developmental phenomena exhibited by those parasites that occasion fatal helminthiases; and, in this relation, I have not confined my remarks to the parasites that are injurious to man in a direct manner, but have extended my observations to the genesis of those entozoa that prove destructive to horses, to beasts of burden generally, and to other creatures which, like cats and dogs, are in various ways subservient to man’s wants. It will be seen that in this way several questions relating to the purity of water and flesh-food, respectively, have been incidentally brought under notice.