84 



„symbiotic^' association of SpongilUdes and alga offers much less 

 advantage to the alga than a life free in the water, as in the sponge 

 all algae are destroyed. In light, on the contrary, that „symbiotic'^ 

 association offers more advantage to the alga than a life free in 

 the water; but that advantage only consists in the fact, that the 

 sponge protects the cdga against destruction, by enemies for in- 

 stance. The milieu — the feeding milieu — , on the contrary, is 

 in the sjwnge not at all more favourable to the alga than in the 

 water, neither in light nor in darkness, but about just as favour- 

 able or even less favourable. When further we know, that also in 

 light algae are constantly destroyed in the sponge — though less 

 than in the water — we must conclude, that from the point of 

 view of the use to the alga that association with the sponge cannot 

 be called at all a symbiosis in the meaning of that of the Lichens. 

 In light the alga, so to say, has chosen the least of two evils ; 

 one cannot say more. 



We have now come to the still more complicate question of the 

 use of the „symMotic^^ association to the sponge. 



I will begin with mentioning all the facts regarding this question. 

 After that we shall see, if all these different data can be united 

 into one conception. 



1. On p. 25 I have mentioned already that the green sym- 

 biotic algae of the Spongillidae lodge oildrops, which they form, 

 as p. 20 shows, by photosynthesis. 



2. The colourless algae may also show oildrops (p. 36); but 

 in general not so often as the green algae; while of these co- 

 lourless ones those, which kept their internal structure, show 

 them more often than the colourless ones, the structure of which 

 has got lost. This may appear from the following analyses of 

 green Spongillae taken from the light, by which the number of 

 algae with oildroplets is given per 100 algae which have been 

 tested. 



