118 



(Fig. 52) ; from which we see that „freie Zellbildung" does not 

 take place, but simple vegetative division of the whole cell. 

 Though I have not been able to give sufficient time to the 

 research into this alga either, I came to the conclusion that we 

 have probably to do with a Pleurococcacea, so with a relative 

 of the ordinary „symbiotic" alga of the fresh- water sponges. 



Besides the 3 „infecting" algae mentioned here, there were 

 of course also the normal „symbiotic" algae in the tissue of the 

 „infected" sponges, but their number was always relatively small. 

 It stands to reason, that there might be in general inside the 

 sponges — in the canal-system for instance — also quite other 

 kinds of algae as well as protozoa and also diatoms, which might 

 of course be captured at their turn by the sponge tissue to serve 

 as food, and so might get into the amoebocytes. But this cate- 

 gory does not come into consideration here at all. Here we have 

 only to do with the algae, which appear and keep up either 

 regularly (the symbiotic alga) or accidentally (the 3 infecting 

 algae) in a great number within the sponge tissue. 



B. THE CURRENT OF WATER IN THE CANAL-SYSTEM 

 OF THE FRESH- WATER SPONGES. 



As mentioned in the Introduction, I have found out a method, 

 which enables us to observe wholly intact, normally living tissue 

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

 consecutive days. The way in which the necessary microscopic 

 preparations were obtained is indicated above on pag. 12 — 13. 

 It was by means of these living preparations that I have been 

 able to state, that the generally acknowledged theory concerning 

 the cause of the current of water through the sponge body is 

 not right, as it is based on a mode of movement of the flagella 

 of the choanocytes, which proved to me abnormal and caused by 

 exhaustion. 



Anatomy. — Before proceeding to the examination of the 



