138 



1899 aud Sollas (53) 190G, either of thom in thoir own way, 

 must have feit something of the theory, described and proved 

 hcre, of the movement of water in sponges. Sollas is nearer the 

 truth than both the other invcstigators ; none of them, however, 

 gives proofs or even mentions cxperiments. 



C. THE INGESTION OF FOOD IN THE FRESH- WATER 



SPONCIES. 



Precediug: Researches. — The question of the ingestion of 

 food is closely related 'to that of the water-current ; therefore 

 the investigators have usually studied them together. 



When studying this problem one should discern the ingestion 

 of solid food from the feeding upon substances in sohition in the 

 w^ater. 



According to Putter (48, 49) 1909 and 1914, it would be 

 absolutely impossible that a sponge feeds on solid food only; on 

 the contrary, it would be more likely that it feeds on organic 

 substances in solution, which would be present in the water in 

 (relatively) large quantities and would diffuse through its surface 

 into its tissues. I shall not speak now about the — to my 

 opinion exact — critic on Pütter's theory by Biedermann (6) 

 and LiPSCHÜTZ (40). I myself have never found a proof of its 

 exactness during my investigations ; on the contrary, the argu- 

 ment on which this theory its based (the deficit of solid food) 

 seems rather not binding for the (green) fresh-water sponges — 

 see pag. 96. But, of course, I do not think the possibility 

 entirely excluded, that a sponge absorbs also feeding substances 

 in solution. Already Haeckel (25) 1872 thought this probable. 

 On the other hand, however, one certainly has no right to con- 

 sider the research of Loisel (41) 1898, who saw vital staining 

 Solutions taken up by sponge tissue, as a proof that sponges may 

 really take up feeding substances in solution, as Minchin (45) and 

 Sollas (53) do, and also Topsent (57) to a certain extent. For 

 we know from the researches of O verton (1902) that, although 



