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alga for such a „relation", but that it would content itself with 

 all sorts of other unicellular algae, according to circumstances. 

 So for instance, in the case mentioned above, with a relative of 

 the normal „symbiotic" alga. In this way one might think possible, 

 in different countries, an association of our fresh- water sponge 

 with quite different algae ! 



IX. Some other algae occurring in the tissues op Ephydatia. 



After the theoretical consideration to which these cases gave 

 reason I want to give now a short description with illustrations 

 of the 3 infecting filamentous and unicellular algae which I have 

 found. I must add that I have met with these algae not once, 

 but in several specimina of Ephydatia (never in Spongillafi) ; but 

 only in one spring, and never in sponges immediately from nature, 

 but always in specimina which had been in my aquaria for some 

 time (2 — 3 months). Generally all 3 of them were to be found 

 together in one sponge, but the unicellular alga also very often alone. 



The Ephydatia a infected by filamentous algae were to be re- 

 cognized at dark-green, almost emerald irregular spots here and 

 there in the colourless or light-green normal sponge tissue. The 

 sponges were also (partly) surrounded at the outside by those 

 algae, which freely spread in the water. Under binocular micros- 

 cope one could observe very distinctly that those green spots 

 might be continued till within the sponge body, so under the 

 normal tissue; but generally they were to be found in tissue 

 layers more at the surface. 



If one examined a piece of such a green tissue-spot under 

 oil-immersion, one found parts, where nothing but the skeleton 

 of the sponge tissue had remained, but the place of the cells was 

 entirely filled in by filamentous algae ; and next to that some 

 almost intact sponge tissue parts, in which however among the 

 normal sponge cells filamentous algae began to spread. One could 

 also see how such a growing algal filament quite makes an 

 exterior wall of the sponge protude (tent shaped), when trying 

 to pierce it from within. 



