The museum of the Royal College of Surgeons of England contains some of the finest specimens of hydatids from the lower animals that are to be seen anywhere, the very choicest of them having been selected by Hunter himself. That distinguished man sought information from every available source, and hydatids were for him of almost equal interest, whether found in the body of a human being or in the carcase of an ox or an ass. Now, at all events, neither pathologists nor sanitarians can well afford to neglect comparative pathology; and, for myself, I am free to say that the yearly exposition to the students of the Royal Veterinary College of the phenomena of parasitic life amongst animals has brought with it an ever-increasing knowledge of the most curious and often unlooked-for information. Some of the data thus supplied are quite remarkable. Let me also add that my studies of the entozoa of wild animals have put me in possession of particulars of high value in regard to the larger question of the origin of epidemics. Beasts, birds, reptiles and fishes, of every description, are liable to succumb to internal parasites, and there is practically no end to the variety of useful information to be obtained from this source. I have collected materials almost sufficient for a separate treatise on this department of the subject, but I fear I shall never have either the time or opportunity to give the facts due publicity. Here, for obvious reasons, I must for the most part restrict myself to the hydatids properly so called.