AUTHOR'S PREFACE 



so forth, will be treated in another monograph of this 

 series, one by Dr. A. JJ. Moore. It is also to be remarked 

 that in Chapter Xlll <>!' the present volume, which deals 

 with the hydroids, the account of the activities of these 

 animals is rather more detailed than that accorded to 

 other forms. This method of treatment has been followed 

 in consequence of the fact that the details of the re- 

 sponses of hydroids are by no means so readily available 

 in the literature as are those of other groups of 

 ccelenterates. 



The writer is indebted to his wife for a full revision 

 of the manuscript and to the editors of the series for 

 many helpful suggestions. He is also under obligations 

 to Mrs. Donald E. Woodbridge for the preparation of 

 the illustrations. 



G. H. P. 



HARVARD UNIVERSITY, CAMBRIDGE, MASS. 

 NOVEMBER, 1918. 



