PREFACE TO THE THIRD EDITION. 



The favourable reception granted to the former editions 

 of this book has already led to a demand for a third. I 

 have again endeavoured to take advantage of the sug- 

 gestions of kindly critics, especially Professor W. C. 

 M'Intosh, Dr. Ramsay Traquair, Dr. John Beard, and 

 Dr. Arthur Masterman. I must also acknowledge grate- 

 fully that in the revision I have had throughout the able 

 assistance of Miss Marion Newbigin, D.Sc, who has also 

 written the chapter on Comparative Physiology. 



The book is intended to serve as a manual which 

 students of Zoology may use in the lecture room, museum, 

 and laboratory, and as an accompaniment to several well- 

 known works, cited in the Appendix, most of which 

 follow other modes of treatment. 



To numerous authorities I acknowledge an obvious 

 indebtedness, a detailed recognition of which would be 

 out of place in a book of this kind. 



Many new figures have been added, and I wish to 

 express my thanks to my artist friends, Mr. William Smith 

 and Miss Florence Newbigin, for the carefulness with 



