1065 



prove: 1^^. Why spongillidae in nature must possess in light and 

 in darkness an amount of the different stages (green ahd colourless) 

 of the symbiotic algae, as we have found by analysis (see above). 

 2"^^. In what way these sponges maintain their "colour" (green or 

 colourless). 3'<^. In what way the two "colour" types pass into one 

 another. It would be beyond the scope of this paper to work out 

 these proofs here. I will only remark, that what happens under 

 certain conditions with the number of green algae of a sponge, in 

 other words, how the "colour" of the sponge is affected, depends 

 wholly on the value, which each of the above 6 factors in these 

 conditions assumes in the formula 



i + wiM -\- r ^ e -\- g -\--mo. 



Thus, if the left side equals the right, then the number of green 

 algae, so the colour of the sponge, remains constant ; if the left 

 side is greater, then their number increases and the sponge 

 becomes greener; if it is smaller, then the number decreases and 

 the sponge becomes colourless. 



IV. The general conception (of course apart from that of Lan- 

 KESTER etc.) of the symbiotic relation of fresh-water sponge and alga 

 is, that it is one probably based upon mutual usefulness. So spongilla 

 counts as almost as classic an example of symbiosis, as the lichens. 

 Yet on the mutual relation of "host" and "guest" but very few 

 experiments, and those not conclusive, have been made by Brandt 

 (I.e.), as BiEUERMANN ^) rightly observes. 



a. I myself have become convinced, by a comparison of the be- 

 haviour of the "symbiotic" algae wiien cultivated (in light) in sponge- 

 tissue and isolated in water, that the "symbiotic" association of 

 sponge and alga offers to the alga more advantage than a free existence 

 in the water. I have been more especially led to this view by com- 

 paring the intensity of multiplication of the algae, as a measure of 

 the favourableness of, briefly, the feeding medium, and by comparing 

 the total increase or decrease of their whole culture, as a measure 

 of the favourableness of all the conditions together. That advantage, 

 however, only consists in the fact that the sponge protects the 

 alga against foes. The feeding medium on the other hand is in 

 the sponge certainly not more favourable to the alga than in the 

 water. Since further we know that the algae are also continu- 

 ally being destroyed inside the sponge, although less so than in the 



1) In Winterstein's Handh. d. Vergleich. Pliysiologie, W, 1911. 



