134 



connectioii with the placing of the prosopyles among the choano- 

 cytes prevents any ovitflow by this last way. On the other hand 

 the low negative pressure, that rnles at the base of the flagella 

 and consequently in the whole zone of the cell-bodies of the 

 choanocytes, must suck iip the water from as near as possible, 

 so here through the prosopyles 5 while moreover all water dis- 

 placement from the centre of the chamber, therefore also from 

 the apopyle, to the base of the flagella is excluded by the action 

 of the latter (perhaps except on the apopylar edge of a choano- 

 cytic" layer, so there, where this one passes into the pinacocytic 

 covering; but this water displacement is either of little impor- 

 tance (Homocoela) or measures must have been taken against it 

 — • more about this later on). 



So in fact we must be able to distinguish always three sharply 

 separated pavts in each acting flagellated chamber with regard 

 to its contents of water: Ist the zone of the negative pressure, 

 that is the zone of the cell-bodies of the choanocytes with the 

 prosopyles; 2n'l the zone of the flagellar fuuction, so the zone in 

 which the water-pressure is increased from negative to positive; 

 3''d the zone of the positive pressure, that is — let me say so 

 for the present — ■ the centre of the chamber with the apopyle. 

 All passing of water from the S'"! into the l^t zone must be ab- 

 solutely excluded ; of course also immediate passing from the 

 l^t into the 3'''i; but passing from the l^t into the 2°^ and from 

 the 2'"^ into the 3''<i zone must certainly take place, and that as 

 quickly as possible. 



Besides the above mentioned relative width of the in- and 

 excurrent openings and the situation of the former among the 

 collar-cells, also the situation of the choanocytes with respect to 

 the excurrent opening must be considered as an important factor 

 for a good regulation of the water-current within a flagellated 

 chamber. The movements of the flagella together give a certain 

 direction to this current, so if there is an opening in the wall 

 of a chamber exactly in the direction of that current, it is in- 

 evitable that the water will flow out by it. The importance of 

 this factor appcars very clearly from both these cases: the first. 



