The object of the writer of the present work has been twofold: first, to lay before the Naturalist a complete view of the organization and physiological relations of every class of living beings; and secondly, to offer to the Anatomical Student a succinct account of the structure and development of the vital organs through all the modifications they present in the long series of the animal creation.

Such were the intentions of the Author, as announced at the commencement of his undertaking; and the reception the first edition received at the hands of the public has been such as to afford gratifying proof that his efforts to facilitate the progress of the cultivators of a science the importance of which is becoming every day more conspicuous have not been unsuccessful.

Since the publication of the preceding edition, however, great and important advances have been made in our knowledge: many and earnest have been the labourers in this enticing field, and proportionately encouraging have been the results. The indefatigable industry of Professor Owen, conspicuous in every department of our science, has, by his invaluable analysis of the vertebrate skeleton, not only re-modelled the nomenclature of the osteologist, but placed in the hands of the Geological Student a light wherewith to guide his steps amid the darkness of departed worlds. The improvements in our microscopes, and the zeal of our microscopists, have much advanced our knowledge of the Infusorial organisms. The researches of Van Beneden and Siebold relative to the embryogeny of parasitic worms open before us a new field of research; while the observations of Steenstrup, Dalyell, and Agassiz, on the "alternation of generations" among the Hydriform Polyps and Acalephae, promise results of the utmost interest to the Naturalist.

The discoveries of Milne-Edwards have importantly increased our information concerning the organization of the Mollusca as well as of the Alcyonoid Polyps; and those of Muller, revealing the metamorphoses of the Echinodermata, add new lustre to a name already so distinguished in science.

To particularize our own countrymen and fellow-labourers whose names give value to the following pages would be an invidious task; suffice it to say that the Author has endeavoured, to the best of his ability, to keep pace with their diligence and onward progress, so as adequately to record and acknowledge their contributions to the general stock of scientific lore.

To Mr. Van Voorst, the liberal Publisher of the present volume, the Author cannot but offer his best thanks; the numerous and costly illustrations that adorn the work speak for themselves, while his endeavours to publish it at a price placing it within the reach of every student will, it is hoped, be extensively appreciated.

Preface To The Third Edition

The short interval which has elapsed since the preceding preface was written affords additional evidence of the increasing usefulness of this work. Encouraged by such success, the. Author, in revising the present edition, has supplied sundry omissions, and added such new observations as the onward progress of anatomical science seemed to require. Important alterations in the arrangement of the Animal Series have likewise been introduced, among which may be pointed out the complete separation of the Protozoa from the Ciliated Infusoria, the introduction and redistribution of the class Helminthozoa, the transference of the classes Rotifera and Cirrhopoda into close proximity with the Crustacea, to which they are related in many particulars of their economy, and the establishment of the Polyzoa as legitimate members of the Molluscous division of Creation. As a general rule, however, the Author has been careful to avoid unnecessary changes in zoological classification, from a conviction that they are rather calculated to embarrass than to facilitate the progress of the student of Comparative Anatomy.