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PREFACE TO THE EIGHTH EDITION 



In this new edition more account has been taken of the 

 physiological aspect, and in this connection I have been 

 helped by my son, Dr. David Landsborough Thomson. 

 More space has also been devoted to the structure and 

 development of certain types, such as the Mud-fishes, 

 which were too briefly discussed in previous editions ; and 

 here I have been indebted to the assistance generously 

 given by Mr. R. M. Neill, M.C., M.A., Lecturer on 

 Zoology in this University. I have also to thank Prof. 

 W. Rae Sherriffs, D.Sc, for many useful suggestions. 

 Numerous figures have been added, mostly drawn from 

 specimens. 



J. A. T. 



The University, 



Aberdeen, August 1929. 



