52 



B. The. factor of export. 



I treat this factor as theoretically possible and the reverse 

 of the import; for a sponge laden with algae might continually 

 eject some of them ; though the above stated eagerness with 

 which a sponge captures, algae does not make this supposition 

 very likely. But I should mention that I have really observed 

 sometimes (not often !) the ejecting of green symbiotic algae' 

 together with feces by vaciioles in the process of defecation, I 

 shall treat afterwards (chap. E). The export can not be expe- 

 rimentally stated in another way ; for a sponge in captivity may 

 always show some small parts of its tissues dying (at least this 

 possibility can never be excluded), whereby algae are set at liberty. 

 I thmk we may consider the export of algae from the sponge 

 tissues as an uncertam^ but prohahly not importa^it factor. 



C. The factor of reduction. 



After some time in captivity the fresh-water sponges show a 

 reduction of thcir tissues, which originally filled the whole ske- 

 leton but then begin to reduce to the inner parts. This may be 

 the consequence of a progressive dying of the outer layers, fol- 

 iowed by tlieir destruction. At least this is often the case. A 

 same phenomenon of reduction, however, might also be caused by 

 the tissues growing more and more compact, by the internal ca- 

 nals and lacunes being filled. It seemed to me that in a sponge 

 in reduction always both processes were going on. Their influence 

 on the remaining paj't of the tissues wonld he the same: the 

 numher of the algae present ivithin the unit of volume o f these tissues 

 ivoiild increase. When the reduction is caused by the tissue growing 

 more compact, this result is quite evident. In the other case we 

 may distinguish two possibilities: 1. The dying amoebocytes might 

 be (partly) ingested by the remaining ones, with preservation of 

 the green algae. 3. The destruction of the dead amoebocytes would 

 set numerous green algae at liberty in the neighbourhood of the 

 remaining sponge parts; thus the factor of import would be in- 

 creased. One can often observe this last phenomenon: a dying sponge 

 is surrounded by a green cover of algae, settled to the bottom. 



