8. 



These are the chief points of my investigation. I want to de- 

 clare emphatically that for all my microscopic examinations I 

 used no other than living preparations. Observing them has 

 been an inexhaustible source of enjoyment to me, also beyond the 

 problems I was about! This makes me think: Those zoologists, 

 who devote" themselves exclusively to morphology, should con- 

 sider that, when studying biological problems, they generally 

 commence by depriving the organism to be examined of the 

 most interesting phenomenon, it ever may show — even the 

 most interesting phenomenon on earth ! — : Life. 



METHODS. 



The capturing of sponges. The sponges were collected in the 

 lakes near Leyden (Brasemermeer). Both forms, Spongilla la- 

 custris and Ephydatia fluviatilis, are to be found there in great 

 quantities; of course, such an unlimited quantity of proof-material 

 is of the greatest importance for physiological investigations. 

 When loosened from their supporting layer — stone or wood — 

 the sponges were immediately cleaned from adhering parts of 

 mud etc. , and transported to the laboratory as quickly as 

 possible. 



The conditions of life in their habitat as to light, purity of 

 the water etc., in connection with the peculiarities of the fresh- 

 water sponges, were studied by me in all seasons for several years. 

 So I got many data, concerning the sponges living under normal 

 conditions, on which I could test the exactness of the results 

 of my proofs. 



The culture of the sponges. After many experiments the fol- 

 lowing arrangement of the aquaj-ia has proved to be the best. 

 The glass aquaria (capacity 50 and 100 litres), and the other 

 vessels used for the experiments, were standing in an unwarmed 

 apartment of the laboratory (windows on S. W.), while daylight 

 was tempered by partly closing the shutters and the air refreshed 



