CONTENTS. 



page 



INTRODUCTION 3 



METHODS • 8 



RESEARCH 45 



A. The chlorophylt, of the fresh-water sponges 15 



I. How and where the green colouring-matter occurs ... 15 



II. The behaviour of the green colouring-matter 17 



III. The nature and structure of the green chlorophyll cor- 

 puscles 21 



IV. The genus of the «symbiotic" alga; its mode of reproduction. 29 

 V. Green and colourless fresh-water sponges; under what cir- 

 cumstances they occur; the nature of their «symbiotic" 

 algae 35 



VI. The structure of the colourless ))symbiotic" algae; how they 



arise from the green ones 36 



VII. The intrinsic amount of the various green and colourless 



stages of the «symbiotic" algae in sponges. The factors 



ruling this amount. How green and colourless sponges keep 



up their «colour", and how they arise from each other . 46 



VIII. The nature of the «symbiotic" association of sponge and 



green alga; its use to the alga and to the sponge. ... 76 

 IX. Some other algae occurring in tissues of Ephydatia . . . 116 



B. The current of water in the canal-system of the fresh- 

 water sponges 118 



C. The ingestion of food in the fresh-water sponges 138 



D. The digestion in the fresh-water sponges 157 



E. The defecation and excretion in the fresh-water sponges . 158 



F. Appendix. Some separate observations 171 



SUMMARY 176 



BIBLIOGRAPHY 186 



TABLES 1—15 189 



ILLUSTRATIONS 217 



Explanation of plates 217 



Plates. I— VI; Figs. 1—77. 



