115 



be some reason to accept, that here we should have to do with 

 a true reaction of defence of the sponge against the foreign in- 

 truder, in the way an unhealthy organism defends itself against 

 the infector. 



I have found : 3 different types of algae (2 filamentous and 1 

 unicellular), which appeared to occur in great number in the 

 tissues of several Ephydatiae (from the Brasemer lake near Leyden 

 and cultivated in my aquaria) (see chapter IX). Two of these, 

 the filamentous algae, proved to destroy entirely the sponge tissue 

 with their growth ; the 3i"d, the unicellular one, on the contrary, 

 after having penetrated the sponge tissue in the beginning — the 

 original colourless sponge had become light-green by it — was 

 finally conquered and destroyed by the sponge. 



In the first place one could imagine this destroying as caused 

 by ferments of defence of the sponge, the above mentioned „poi- 

 sonous" influence of products of metabolism. But there is still 

 another possibility. It proved to me, that a sponge may eject 

 such unicellular „infecting" algae from its body together with 

 detritus (for instance captured carmine grains). (See lateron, in 

 the description of the defecation-process, chapt. E.) 



So here we have seen cases of positive infection of the sponge 

 by filamentous algae, by which the sponge was destroyed. But 

 we also saw a case, in which the sponge finally succeeded in 

 conquering and destroying the intruder — the unicellular alga — 

 which was spreading further and further. 



That unicellular alga appeared to be probably a Pleurococcacea, 

 so closely related to the „symbiotic" alga of the Spongillidae 

 (p. 34). Is it not evident then, that one is inclined to see also 

 something in that „symbiosis", which is like an infection? An 

 infection, against which the sponge must also defend itself? 



There is still another view in connection with the case of the 

 mentioned unicellular alga. If the „relation" between sponge and 

 „symbiotic" alga is no other than was mentioned (p. Ill — 113), 

 it might be possible, that the sponge does not want one certain 



