113 



and then not even always {}). 41^ 75 — 76). If, however, the circum- 

 stances are somewhat less favourable — as is the rule in sponges 

 in darkness {p. 69 — 70) and as sometimes happens also in those 

 in light {p. 41, 75 — 76) — then all imported algae {and all that 

 might he present already) are continually and unavoidably destroyed 

 and digested by the sponge {p. 111). 



What happens to the number of green algae of a sponge under 

 certain circumstances, entirely depends upon the value, which each 

 quantity takes under those circumstances in this formula: 



i -\- r -\- mu =:^e -\- g -\- mo 



the formula, which we have got to know on p. 68 — 75 as decisive 

 for the number of green algae of a sponge. 



For the „symbiosis" considered from the point of view of the 

 use to the alga, I refer to p. 83 — 84. 



In order to show even more clearly, how much the relation 

 of fresh-water sponge and green alga is still removed from a real 

 symbiosis (in the sense of mutualism), I want to mention what 

 we should require from a relation between two organisms, which 

 are closely connected, to be justified in calling this relation a 

 symbiosis (mutualism) : That relation should be one of mutual 

 use; the symbiontes should be interested in each others 

 existence, so spare, if possible, even nourish each other. Both 

 the symbiontes should in fact behave as one, new individual — 

 in extreme cases, for instance, not be able to exist one without 

 the other and die together. The symbiosis should be kept up 

 simply by multiplication of both symbiontes 5 but should not need 

 a continual supply from outside (import) of one of the symbion- 

 tes, to restitute the destroyed ones. 



So Noll (56) says about the Lichens : „Die Pilzhyphen umspin- 

 nen im Flechtenkörper die Algen, überlassen ihnen den zur As- 

 similation giiiistigen Platz . . . . , treten mit ihnen in innige Be- 

 rührung und entziehen ihnen einen Teil ihrer Assimilate. Dafür 



