53 



In nature, however, the process of reduction occurs in autumn 

 only. We may therefore neglect this factor for sponges living 

 free in nature (except in autumn) ; but we should prevent that 

 in our experiments (in aquaria etc.) this reduction becomes effi- 

 cient, and we should take care to limit its troubling consequences 

 (the increase of the import) as much as possible by a continual 

 vigorous circulation of fresh-water through the aquaria. 



D. The factor of growth. 



As a very young sponge forms but a minute corpuscle, when 

 full-grown, however, a large crust with long branches (10 c.M. 

 and even longer) — Spongilla — or a thick cushion — Ephy- 

 datia — , it stands to reason that the growth of the sponge must 

 be (not in a short, hut in a long space of time) a very active 

 factor of decreasing the number of algae present in the unit of 

 volume of its tissues. 



One should consider, however, that growth takes place almost . 

 exclusively at the top of the branches in Spongilla, as I men- 

 tioned already , on pag. 16. So especially here the factor of 

 growth will act, although it might be possible that this in- 

 fluence is spread all over the sponge tissue by means of an ac- 

 cumulation of amoebocytes from all parts of the sponge body to 

 the branch tops. But it is remarkable to notice that these tops 

 are exactly the parts of a sponge, which become green first of 

 all and remain so for the longest time, as I stated repeatedly ; 

 so some other factors must act here as well. Nevertheless, I have 

 once found in a very warm season a great number of quickly 

 grown, green Spongillae with indeed very light-green coloured tops. 



Green Spongillae (from light) proved to be larger than colour- 

 less ones (from darkness). This difference can not always be ob- 

 served, of course; nor is it always so prominently marked out as 

 in Fig. 1 and 2 ; but I have been able to state it in general and 

 in different months of several years. Later on I will treat the 

 cause of this phenomenon (chap. VIII). So we may conclude the 

 factor of growth to be more active in green sponges in light than 

 in colourless ones in darkness. 



