137 



the canal-system of the fresh-ivater sponges is caused hy the fiageUar 

 motion of the choanocytes in the flagellated chambers. This motion 

 {in normal condition) takes place in a spiral- or an undulating- 

 li?ie^ namely in a very rapid succession of ivaves of small ampli- 

 tude^ passing along the flagellum from the base to the top {Fig. 56'a, 

 59) \ by which a current of water arises straight through the axis 

 of the flagellar spiral^ and sitnilarly in the direction from base to 

 top, while the water flous on at the side of the base {Fig. 56a). 

 Exhaustion causes quite different motions of the flagellum, with 

 abnormal current of water {Fig. 56 b-d). The whole water-current 

 within a flagellated chamber is of course the resultant of the little 

 currents caused by each flagellum separately; it is rapid and re- 

 gular {Fig. 63). In order that a poiverful and steady current may 

 be maintained by the chamber, and that, therefore, the water will 

 flow in qiiickiy and exclusively at the prosopyles and flow out by 

 the apopyle, the structure of the flagellated chamber must comply 

 with definite requirements. This structure must be such that: 



a. the 3 zones, which can be distinguished in a functionating 

 chamber, P^ the zone of negative, 5«ö^ that of increasing, 

 3^'^ that of positive water-pressure, remain absolutely separated, 

 so that 710 water can pass from one zone into the other in 

 any other way than from the P^ into the 2"^ and from the 

 2»d into the 3^d zone (Fig. 63). 



and that: 



b. this way of passing of the water goes as quickly as possible 

 (Fig. 64). 



Finally some separate points: 



The motion of the flagella in the chambers does not chang-e 

 when the ostia close, as I repeatedly stated. 



Besides the above (p. 135) mentioned function of regulation 

 of the current, we can probably ascribe to the collars that of 

 protection of the flagella against injury and mutual entanglement. 

 A third and much more important function will be treated in 

 the next chapter. 



Finally I should mention that Delage and Hérouakd (16) 



