The reader who is sufficiently acquainted with the progress in vertebrate physiology during the last phase of physiological methods, and who knows how scattered and incomplete are the investigations which have been made by the same kind of physical and chemical inquiries on invertebrate animals, will not expect to find in the present volume any complete statement of the physiology of animals, in the sense in which that term is now used. Such observations as have been made without especial reference to the vital processes of man are, for the most part, very valuable and suggestive but the time to write a text- book of Comparative Physiology, as we now understand it, has not yet arrived. All that I have attempted to do in this little book has been to illustrate the details of structure by a notice of such experimental inquiries as I have convinced myself, or have adequate reason to believe, are, in their broad outlines, correctly stated. I have much more attempted to make use of what were long since called the experiments that Nature makes for us, by referring to, sometimes perhaps insisting on, the different methods by which similar results are attained by different animals. That which I have most constantly kept before myself, and which I hope the student will faithfully bear in mind, is, that there has been an evolution of organs as well as of animals, and that he who desires to understand the most complicated organs must first know the structure of such as are more simply constituted. In pursuit of this object, I have written about organs rather than about groups of animals but I have added an index in which the various parts of an animal are collected under the head of its name so that the student who desires to use this manual as a zoological text-book will have no difficulty in selecting the portions of the chapters which bear on a particular form or set of forms. I have departed a little from the ordinary method of writing a handbook, in somewhat plentifully interspersing the names of my authorities for various statements. I have done this, not only because it recommends itself to my sense of justice, but becau.se zoological science is just now advancing so rapidly that many observations and suggestions have to be incorporated, even in a text-book, before they become the general property of zoological workers.