15 



space in which they oocur, as well as by their greatest number, 

 we know as normally occurring within that space. But, as men- 

 tioned above, we will never have to compare mutually direct 

 the numerousness in the different groups (that of the chloro- 

 phyll corpuscles, that of the oildrops, etc.), but only the chan- 

 ges of number — i. e. the increase or decrease — in the 

 different groups mutually. That can always be done, of course. 



RESEARCH. 

 A. THE CHLOROPHYLL OF THE FRESH-WATER SPONGES. 



I. How AND WHERE THE GREEN COLOURING-MATTER OCCURS. 



Spongilla lacustris and Ephydatia fluviatilis — the only fresh- 

 water sponges I am going to treat in this paper — occur in our 

 country in a green and in a colourless form ; between which, 

 however, exist many intermediate tints from emerald-green to 

 creamy-white. But a newly caught specimen never shows any 

 of these colours purely ; for, in consequence of its growth in the 

 more or less dirtied water of our canals and lakes, the sponge 

 tissue is so overloaded with particles taken from the water, 

 that the colour may have taken a dirty-brownish tint. The green 

 sponges do not show this very clearly, but creamy-white speci- 

 mina are even never to be found : they are always gray -brown 

 in different variegations. I therefore think that all the different 

 colours of the fresh-water sponges mentioned in literature (eme- 

 rald-green, green, brown, yellow-brown, flesh-coloured, gray, 

 dirty-white, white) are simply to be reduced to the principal 

 colours (grass-)green and creamy-white with their intermediates, 

 while the others can be explained as having been caused by the 

 dirtied water, in 'which the sponges were living. 



I conclude this from the following facts : 1. I collected my 

 sponges in a moor-lake, with pale brown coloured water, contain- 



