Originally published in 1951 as part of the Cambridge Library of Modern Science, this book discusses the life cycles of various parasitic animals, as distinct from parasitic bacteria and plants. Key biological points are illustrated with references to parasitic animals that attack either humans or domestic animals for the purposes of clarity, and the effect of the host-parasite relationship on both parties is also examined. This book will be of value to anyone with an interest in the history of science and parasitology.