Reactions of Sponges 25 



cytes, like those of the osculum, are cells not inappropriately 

 described as primitive smooth muscle-fibers. 



In water composed of three-quarters seawater to one-quarter 

 freshwater, the ostia remained open, and a strong ostial current 

 could be seen at the end of twenty minutes. When the ostia 

 were closed the effect of this mixture was to induce an opening 

 of the ostia in about a quarter of an hour, after which a strong 

 ostial current continued to flow. To mixtures of half seawater 

 and half freshwater oscular, and sub-dermal currents as well as 

 ostial currents ceased in about ten to twelve minutes, showing 

 that though the ostia probably remained open, the currents 

 ceased because of the collapse of the choanocytes. Closed ostia 

 in this mixture opened slightly and then all currents ceased in 

 from nine to ten minutes. In fresh water all currents ceased 

 immediately. These mixtures of seawater and freshwater so far 

 as their effects can be seen, influenced the ostia much as they did 

 the oscula, in that they induce a partial but imperfect contrac- 

 tion. 



In seawater rendered free from oxygen by boiling and subse- 

 quently cooled to 28° C, the ostia remained open or, if closed, 

 they opened in from seven to ten minutes. This reaction was 

 precisely the reverse of that of the osculum and to make close com- 

 parisons of the two I prepared several fingers of Stylotella by 

 cutting them off rather short at the proximal ends thus perman- 

 ently opening the gastral cavity and by leaving the oscular end 

 intact. These preparations were placed one after another under 

 the microscope in pure running seawater and after the oscula 

 were freely open the current of pure seawater was changed for 

 one of deoxygenated seawater. Under these conditions all the 

 oscula closed in about ten minutes, if not completely at least 

 nearly so, and the ostia remained open, their currents now dis- 

 charging chiefly through the cut end of the gastral cavity. Deoxy- 

 genated water, then, is a means of closing the oscula and opening 

 or leaving open the ostia. 



To seawater containing juice expressed from an oyster, either 

 fresh or foul, the open ostia remained open and their currents 

 seemed at times to increase. I was never able to demonstrate 

 with certainty that to these materials the closed ostia would 



