PREFACE. 



The work presented in this paper has been done partly at 

 the Marine Biological Laboratory, Woods Hole. Mass., partly 

 at the Bermuda Biological Station for Research (Contribution 

 from this station No. 140). I wish to express my great indeb- 

 tedness to the directors of these institutions, Dr. Frank R. Lillie 

 and Dr. E. L. Mark, for giving me the opportunity to carry 

 out the work and affording all the facilities which I might 

 have desired. 



It was written down during my military service. If it is too 

 longwinded here and there, my excuse is that of Cicero: 

 Excuse me for writing you a long letter, I did not have 

 time to write an short one" 



