The present book is the outgrowth of a course of lectures delivered during the past six years at the University of Michigan. This course with the accompanying laboratory work was based on the frog, which was chosen as a convenient form with which to introduce students to a knowledge of the morphology, physiology, and life history of vertebrate animals. In writing this book I have had in mind the needs of students, such as most of those taking this course, who have had some preliminary work in general biology, but who have forgotten most of what little of the elements of physiology they may have learned in the schools. A certain amount of physiology of a more or less general nature has accordingly been introduced in addition to the descriptions of the special functions of the various organs of the body. The book is more suitable for use as a text in college or university classes than in high schools, although it is hoped that it will prove of service to teachers in high schools where the frog is studied in the course in zoology.<br><br>Considerable space has been devoted to describing the habits and natural history of the frog, and the endeavor has been made throughout the work to correlate the study of structure with that of the physiological functions of the body and the activities of the organism as a whole in relation to the environment.