Reactions of sponges 21 



by it. When this current was shut off the osculum often closed 

 and under such circumstances, as might have been expected, the 

 ostial currents ceased. To be certain that the cessation of these 

 currents was due to the closure of the outlet, I cut off a closed oscu- 

 lum and found that the ostial current almost im.mediately began 

 again. Moreover when I liga ted the cutoscular end of a finger on 

 which ostial currents could be easil) seen these currents ceased at 

 once and on the removal of the ligature the currents recommenced. 

 From these observations it is quite clear that the osculum con- 

 trols in a purely mechanical way the current within the sponge. 

 When the osculum is open this current may run; when it is closed 

 the current ceases even though the ostia are open and the choano- 

 cytes continue to beat. In view of these facts I regularly removed 

 the oscular ends from fingers of Stylotella on which I wished to test 

 the ostia. 



Although the presence of an ostial current is conclusive evi- 

 dence of the openness of the ostia, its absence is not proof that the 

 ostia are closed even supposing that the oscular end is cutoff, for it 

 is conceivable that the choanocytes may cease to beat, in which 

 case the cessation of the current would be misleading as to the con- 

 dition of the ostia. For some time I was puzzled as to a means of 

 meeting this difficulty, but a simple method finally suggested itself 

 and was adopted. If the oscular end of a finger of Stylotella is 

 cut off at some distance from the osculum, the cut face includes 

 not only the gastral cavity and some of the flagellated chambers, 

 but also the sub-dermal cavities. An examination of the currents 

 from such a cut end will show a large, slow, central current emerg- 

 ing from the gastral cavity and a considerable number of smaller 

 more rapid currents entering the surrounding sub-dermal cavities. 

 These cavities form a set of intercommunicating spaces over the 

 whole surface of the sponge, and the currents that set into them at 

 the cut end depend purely upon the action of the choanocytes. 

 If, now, no inward currents can be detected at the ostia but cur- 

 rents can still be seen to enter the sub-dermal cavities at their cut 

 ends, it is clear that the absence of lateral currents is due to the 

 closure of the ostia and not to the cessation of the choanacytes. 

 In this way, then, I used the presence of ostial currents to indicat 



