130 TOILERS IN THE SEA. 



ones their diameters are reduced to half, or one- 

 third, of what they were while in full action. Simul- 

 taneously with the decline in the force of the out- 

 flowing 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 oscule. 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 poured 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 prompt- 

 ings of its own instinct in the choice of its periods 

 of feeding and repose." 



We may infer that the chief source of nutriment 



