113 



and f hen not even always (p. 41 ^ 75 — 7(i). If^ hoivever^ the circum- 

 stances are mmewhat less favourahle — as is the rule in sponcjes 

 in darkness {]>. (>■) — 70) and as sometimes happens also in those 

 in light {p. 41^ 70 — 76) — then all imported algae {and all that 

 mif/ht be present already) are continually and unavoidahly destroyed 

 and digested hy the sponge {p. 111). 



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

 certain circumstances^ entirely depends upon the value, which each 

 quanfity fakes under those circiimstances in this formula: 



i -\- r + mu =.e -\- g -\- mo 



the formula^ which we have got to know on p. GS — 75 as decisive 

 for the numher of green algae of a sponge. 



For the „symbiosis" considered from the point uf 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 ethers 

 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 ; but should not need 

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

 tes, to restitute the destroyed ones. 



So ï^OLL (56) says about the Lichens: „Die Pilzhyphen umspin- 

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

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

 rührung und entziehen ihnen einen Teil ihrer Assimilate. Dafür 



