lack of light is really the cause of the green (colourless) sponges 

 becoming (remaining) colourless in darkness, but for quite a 

 different — much more complicate — reason than Brandt and 

 Lankester think of (see Summary, point 7 — 15). 



4. The general conception (except of Lankester c. s.) of the 

 symbiotic relation of fresh-water sponge and alga is, as a relation 

 probably based on mutual use. So Spongilla is almost considered 

 to be a „classic" example of symbiosis next to the Lichens. 

 "We possess however but a few proofs, very little decisive, con- 

 cerning this mutual relation of host and guest (Brandt 8). Now, 

 with my experiments I have come to the conclusion that, instead 

 of a classic example of symbiosis in the sense of the mutualism 

 of the Lichens, the association of sponge and alga may be called 

 at best a transition of a process of nutrition (of the sponge) into 

 a still very imperfect symbiosis (see Summary, point 16 — 19). 



These were the original points I intended to examine. But, as 

 is usually the case, when seeking we find by chance quite new 

 ways leading to other territories. So I found out a method, which 

 enabled me to observe wholly intact, normally living tissue 

 of sponges with oil-immersion for many hours, on several consecu- 

 tive days. In this way I got an insight into the following points : 



5. As for the cause of the current of water in the canal-system 

 of the sponges, the research and theory of Vosmaer and Pekel- 

 haring (62) 1898 are almost generally acknowledged — eg. by 

 MiNCHiN (45) 1900, Biedermann (6) 1911, Babak (3) 1912 and 

 partly by Jordan (32) 1913 as well — : By the irregular strokes 

 (to and fro) of the flagella of the choanocytes the pressure of 

 the water on the inside of the wall of the flagellated chambers 

 is continually changing; one time it is positive, next time nega- 

 tive. In the first case the choanocytes acting as valves will pre- 

 vent the water from flowing out through the prosopyles. If, on 

 the contrary, the pressure is lessened, water can easily flow into 

 the chamber by these openings. The sponge will thus suck in 

 water through the incurrent canals, which flows out again by the 

 osculum. The current in the flagellated chambers is not regularly, 



