A CONTRIBUTION TO THE PHYSIOLOGY OF 

 THE FRESH^WATER SPONGES (SPONGILLIDAE). 



PREFACE. 



A paper as follows must necessarily be established by a de- 

 tailed statement of observations as well as by illustrations taken 

 from life. As my investigations have led to so many subjects, 

 this paper has become rather extensive ; and it would have lost 

 its clear survey, had I not taken special precautions to prevent this. 



1. For a survey of the questions and the results, I refer to the 

 various points of the Introduction and to the adjoined points in 

 the Summary at the end of this paper. This Summary gives the 

 chief results, and the pages of the text^ the tables and illustrations 

 concerning them. 



2. In the text itself the questions and results are always printed 

 in italics. 



My principal results have already been published in the Pro- 

 ceedings of the Kon. Akademie van Wetenschappen at Amsterdam, 

 meeting 24 Nov. 1917. 



