50 



duit, does not at all decrease in the course of some weeks but 

 even strongly increases, as stated already on pag. 28. All this 

 concerns Spongilla as well as Ephydatia. (See Table 8.) 



In this way the import of the green algae proved possible. 

 One might ask, however, if this factor really exists in nature 

 under normal conditions. This is certainly the case. For we 

 know that the symbiotic algae occur free in nature in exactly 

 the same waters as the sponge is living in (pag. 27 — 28); namely 

 to the amount of at least 200 per litre in March and at least 

 3700 in July; but these numbers give a minimum only, probably 

 very much too low. Moreover, one should not consider the in- 

 tensity, with wich a sponge filters the surrounding water clear, 

 to be but weak. As to this, we saw in Table 7 that a little 

 sponge (of but 3 c.M^ vol.) is able to make clear 3 litres of a 

 troubled suspension within 3 days, while the filtered water was 

 again and again mixed with the suspension. So, how many times 

 had not the same water to pass through the sponge body before 

 all was clear at last; how many litres has the little sponge really 

 filtered in those 3 days! Another proof of this really enormous 

 „filtering power" : sponges as large as a finger proved able to 

 make clear 3 litres of water mixed with 2 c.M'' of milk within 

 a day's time; but exclusively when oscular tubes were present. 

 (This result was not caused by sponge-enzymes, as one might 

 think ; for such liquids mixed with material from a pressed 

 sponge remained troubled for days. Nor did the milk sink to 

 the bottom). 



Having stated now the presence of green symbiotic algae free 

 in the water and the enormous y, filtering 'poiver'''' of a sjjonge, we 

 may conclude that in nature the import is a vigoronsly and con- 

 tinually acting factor of increasing the number of green algae in 

 sponge-tissues. And this must be the case! For we stated (p. 47) 

 in colourless sponges from darkness that green algae are con- 

 stantly present in a small number, that stages of division are 

 generally absent, that the colourless algae (without structure) in- 

 crease considerably in number, in other words, that green algae 

 are continually dying; so it is absolutely necessary, that new 



