22 G. H. Park 



arker 



that the ostia were open and their absence, when coupled with the 

 presence of sub-dermal currents, to indicate that these apertures 

 were closed. Mostof the tests that were carried out on the oscula 

 were repeated on the ostia and the results will be stated briefly in 

 the following paragraphs. 



a. Mechanical Stimulation 



A finger of Stylotella from which the oscular end has been cutoff 

 may be kept a long time in running seawater in apparently normal 

 condition. When the current of seawater is temporarily shut off, 

 small suspended particles can be seen to drift slowly up to the sur- 

 face of the finger and disappear by suddenly darting into the ostia. 

 In this way the ostia can be demonstrated to be open. Many par- 

 ticles are too large to enter these apertures and they will accumu- 

 late on the surface of the animal in quiet water. When, however, 

 the general current is set going again, it sweeps these larger par- 

 ticles away and leaves the surface of the sponge relatively clean. 

 If after the ostia have been demonstrated to remain open for some 

 time in running seawater, this current is permanently shut off so as 

 to leave the sponge in quiet water, the ostia may con tinue open for 

 many hours during which the surface of the sponge often becomes 

 deeply buried under an accumulation of particles most of which 

 are too large to enter the ostia. In no case has a cessation of the 

 seawater current been followed by a closure of the ostia as with the 

 oscula. The ostia, then, differ from the oscula in that they remain 

 open in both quiet and circulating seawater. 



Prepared fingers of Stylotella in which strong sub-dermal and 

 oscular currents could be seen but no ostial currents were persent, 

 were kept in some instances in running seawater, in others in quiet 

 seawater without, however, yielding any evidence to show that 

 either state of the seawater caused the opening of the ostia. Flow- 

 ing seawater and quiet seawater both seem to have no effect on 

 the opening or closing of the ostia in Stylotella. 



Even when Stylotella is covered with a deep layer of silt, its 

 ostia can often be demonstrated to be open. Under ordinary cir- 

 cumstances, however, it is not usual to find this sponge thus 



