1069 



and is no longer driven awa} (fig. Id). Still 5 minutes later and 

 all movement has ceased, the tlagellnm is extended {le). As I have 

 already said, I have also observed this same succession in intact living 

 flagellated chambers, when in consequence of very prolonged 

 experimenting the choanocytes had got into an unfavourable condition. 

 I should mention, that the isolated choanocyte represented here 

 (fig. 1) did not show a collar; this was therefore evidently wholly 



Fig. 1. Successive stages of the flagellar movement of an isolated choanocyte. 

 The cell-body is still connected with several other choanocytes, the collar is 

 retracted, a immediately after isolation. The arrows indicate the direction of the 

 water-current, the dots floating particles; the moment of observation is given in 

 each case. At e the flagellum has finally come to rest. Magnification about 1770 

 times. 



contracted. In the intact flagellated chambers the collars were how- 

 ever very plainly visible as long cylinders, which left the flagellum 

 uncovered for only a relatively short apical portion (fig. 2 — 5). 



The whole water-current in a flagellated chamber is of course the 

 resultant of the water-displacements caused by each individual flagellum; 

 it is rapid and regular, as can best be made clear by a diagram- 

 matic figure (fig. 2). It is seen, how from the prosopyles (one only 

 is here indicated, but there are 2 — 5 in one flagellated chamber) the 

 water must flow between the cell-bodies and the collars of the 

 choanocytes, rapidly and regularly, towards the base of the flagella 

 (here really the opening of the collars), to t>e forced hence by the 

 flagellar movement towards the middle of the flagellated chamber, 

 thence to stream away through the apopyle. In order that a 

 powerful, steady current may be maintained by the chamber and 

 that, therefore, the water may enter rapidly and exclusively at 

 the prosopyles and flow out by the apopyle, the structure of the 



74 



Proceedings Royal Acad. Amsterdam. Vol. XX 



