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through a pore than it finds in streaming in. For we found that 

 in carefully mounted preparations where all the choanoeytes re- 

 mained fixed in their place, these cells surround the pores closely 

 and are placed, not exactly perpendicular on the wall, but some- 

 what oblique, so as to narrow the cloacal opening of the pore. 

 We found this to be the case in a flat, stretched piece of Leu- 

 cosolenia. Their position raust be all the more oblique in the 

 living state if the wall is of course not flat but concave. If 

 therefore in the cloaca the pressure of the water becomes higher, 

 the collars of the choanoeytes will become somewhat inflated and 

 the pore will be narrowed. If, on the contrary in the cloaca the 

 pression of the water in the neighbourhood of a pore is lessened, 

 water can easily flow in through the pores ; the choanoeytes with 

 their collars thus act as valves. Now by the irregular motion of 

 the flagella the pressure on the wall of the tube, which by the 

 spicules is kept rigid, is continually changing. If the pressure 

 becomes higher, this is of little eff'ect, but if the pressure becomes 

 less, water will flow in through the pores, as long as they are open. 

 The sponge will thus suck iu water, which will leave the body 

 again through the osculum". 



Thus Vosmaer's and Pekelharing's theory. Then the investi- 

 gators call attention to the fact that the arrangement of the canal 

 system of the sponges becomes more and more appropriate accord- 

 ing as one examines higher developed forms. 



Personal Research. — Hoiv much this tJieorij of Vosmaer and 

 Pekelharing's may correspond with their observations and hoiv well 

 it may make us miderstand that ivith such a movement of the tvater 

 a (jreat number of foodpartides are brought uithin the reach of 

 the choanoeytes^ it could not quite satisfy me froni the beginning. 

 It appeared very unlikely to me that the flagellar motion of 

 the choanoeytes should principally be quite different from that of 

 the unicellular Flagellaia eg. the Choanofagellata^ which for the 

 rest remind ns so much of the choanoeytes. This supposition was 

 the more tompting, because it would make us quite independent 

 of the synchronism (whether existing or not) and of the direction 



