Reactions of Sponges 23 



covered. Its natural habitat is in a current of seawater and 

 though this water niav often be heavily loaded with suspended 

 matter, its current seems to be sufficient, as already noted, to 

 remove many particles which, from their size, accumulate on the 

 surface of the sponge. But this is not the only means for the 

 removal of these larger particles. A close inspection of the outer 

 surface of Stylotella will show that it is regularly inhabited by 

 several animals; chief among these are young ophiurans, caprellas, 

 and a species of copepod. All these animals, and especially the 

 copepods, keep up an incessant movement over the surface and 

 loosen and dislodge much of the accumulated drift. The cope- 

 pods and probably the other forms find much to feed on in this 

 omnium-gatherum, and their relation to the Stylotella seems to be 

 of a symbiotic character. These organisms together with sea cur- 

 rents are responsible for the generally clear character of the sur- 

 face of the sponge. But even when this sediment is abundantly 

 present on Stylotella its ostia remain open. I have also failed to 

 find any evidence in favor of the view that when the ostia are 

 closed the accumulation of silt on the surface of the sponge will 

 cause them to open. 



Exposure to air likewise seems to have no effect on the ostia. 

 A finger of Stylotella on which the ostia were freely open was 

 exposed to air for about a quarter of an hour. Upon reimmersing 

 It in seawater the ostial current could be seen at once. It was 

 again put in the air, this time for three-quarters of an hour, where- 

 upon it was reimmersed and the ostia again gave evidence of being 

 freely open though the finger as a whole had the shriveled appear- 

 ance characteristic of sponges that have been exposed sometime to 

 the air. 



Stroking the surface of Stylotella seems neither to bring about a 

 closure nor an opening of the ostia. In this respect they are as 

 irresponsive as the oscula. 



So far as mechanical stimulation is concerned, the ostia are very 

 unlike the oscula. The oscula are responsive to water currents and 

 their absence and the mechanical effects of exposure to the air; the 

 ostia are uninfluenced by any of these changes and are apparently 

 also undisturbed by sediment. 



