PREFACE. 



On account of its clearness and breadth of view, its comparatively 

 simple character and moderate size, Professor Richard Hertwig's 

 'Lehrbnch der Zoologie' has for ten 3'ears held the foremost place 

 in German schools. The first or general part of the work was 

 translated in 1896 by Dr. George W. Field, and the cordial recep- 

 tion which this has had in America has led to the present rejiroduc- 

 tion of the whole. 



This American edition is not an exact translation. With the 

 consent of the author the whole text has been edited and modified 

 in places to accord with American usage. For these changes the 

 translator alone can be held responsible. Some of the alterations 

 are slight, but others are very considerable. Thus tlie group of 

 Vermes of the original has been broken up and its members dis- 

 tributed among several phyla; the account of the Arthrojioda lias 

 been largely rewritten and the classification materially altered ; while 

 the Tunicata and the Enteropneusti have been removed from their 

 position as appendices to the Vermes and united with the Verte- 

 brata to form the phylum Chordata. Other changes, like those in 

 the classification of the lleptilia and the nephridial system of the 

 vertebrates, are of less importance. 



A large number of illustrations have been added, either to make 

 clearer poii^ts of structure or to aid in the identification of American 

 forms. Except in the Protozoa, American genera have in most 

 cases been indicated by an asterisk. Numerous genera have been 

 mentioned so that the student may see the relationships of forms 

 described in morphological litcratui-e. 



In the translation the word Anlage, meaning the embryonic 

 materia] from which an organ or a part is developed, has been 

 transferred directly. As our language is Germanic in its genius, 

 there can be no valid objection to the adoption of the word. 



As this work is intended for beginners, no bibliography has been 

 o-iven. A list of literature to be of much value would have been so 

 large as to materiallv increase the size of the volume. Experience 



