118 ANATOMY AND PHYSIOLOGY 



of the oscula, while in others it is proceeding in its full 

 vigour, and sometimes it will be suddenly renewed for a 

 brief period in those in which it had apparently ceased. 

 These vacillations in the performance of its functions is 

 always indicative of an approaching cessation of its 

 vigorous action. When the vivid expulsion of the vv^ater 

 has ceased, the aspect of the oscula imdergoes a consider- 

 able change; some of the smaller ones gradually close 

 entirely, while in the larger ones their diameters are reduced 

 to half or one third of what they were while in full action. 

 Simultaneously vnth the decline in the force of the excur- 

 rent action the greater portion of the pores are closed, a 

 few only dispersed over the surface of the sponge remaining 

 open to enable the gentle inhalation of the fluid to be 

 continued, which is necessary for the aeration of the 

 breathing surfaces of the sponge. The breathing state of 

 inhalation appears to be very persistent, and I have rarely 

 failed in detecting it when I have let a drop of water, 

 charged with molecules of indigo, quietly sink through the 

 clear fluid immediately above an open osculum. These 

 alternations of repose and action are not dependent on 

 mere mechanical causes, and sponges in a state of quiescence 

 may be readily stimulated to vigorous action by placing 

 them in fresh cool sea-water, and especially if it be pom*ed 

 somewhat roughly into the pan, and agitated briskly for a 

 short period ; and this will take place even in specimens 

 that have very recently been in powerful action. 



No general law seems to guide the animal in the choice 

 of its periods of action and repose, and no two sponges 

 appear to coincide entirely in the time or mode of their 

 actions. In fact each appears to follow the promptings of 

 its own instinct in the choice of its periods of feeding and 

 repose. 



In the httoral sponges there is a third condition of the 

 animal, and that is during its exposure to the atmosphere 

 in the intervals between high and low water, and in some 

 sponges the pores and oscula are both completely closed. 

 But this condition does not obtain in all species. Thus, 

 during the course of my investigations at Tenby, I observed 



