48 



9. In the green sponges the colourless algae are generally less 

 numerous than in the colourless sponges — of course one must com- 

 pare tissues in the saine stage of develop)ment — ; this concerns 

 the colourless algae ui'th structure as well as fhose u-ithout. For 

 the first ones this difference in numher is smaller than for the 

 last ones ; and for the latter it still increases rather considerably 

 during the development of the sponge tissue^ in order to prohahly 

 disappear almost entirely during or Immediately after the gem- 

 mule stage. 



10. In the green and, in the colourless sponges the numher of 

 colourless algae with structure is ahvays much smidler than that 

 of those without structure. 



11. The total numher of symhiotic algae present in the green 

 sponges surpasses that in the colourless ones. 



Having stated this^ we want to know^ what is the reason of 

 all this {point 1—11). In short, why in nature a {green) sponge 

 in light contains an excess of green living algae and a smaller 

 numher of colourless dead ones, a (colourless) sponge in darkness, 

 on the contrary, an excess of colourless dead algae and a smaller 

 numher of green living ones ; how hoth sponge types keep up their 

 „colour'"; a?id how they arise from each other (pag. 35). 



For that purpose we have to examine tJie factors ruling the 

 numher of the symhiotic algae in the sponge tissues. These factors 

 are 6 in number, viz. : 



A. the import of the algae from the surrounding water into 

 the sponge (the factor of import). 



B. the export of the algae from the sponge-tissues into the 

 surrounding water (the factor of export). 



C. the increase of number of the algae in certain parts of the 

 tissues by the reduction or deatli of other parts (the factor 

 of reduction). 



D. the dccreaso of number of the algae in a certain sponge- 

 volume by the growth of the sponge; (the factor of growth). 



