It is perhaps because fishes live in a buoyant medium, whether it be fresh or sea water, that they show a diversity in body shapes that is unparalleled by other vertebrates. There is also a unique diversity in the modes of reproduction, whether by external or internal fertilization, and this, with the morphology and fine structure of the reproductive system and its components, is the subject of the present volume. A large part of this book deals with the ultrastructure of spermatozoa, with some discussion of phylogenetic implications.