FORM OF A SPONGE 



297 



On these two factors depends the hfe of the sponge, or of 

 any other fixed or stationary organism in still water. The 

 outgoing current carries with it water which has been j&ltered 

 of food, in which carbonic acid has been substituted for oxygen, 

 into which the poisonous products of metabolism have been 

 excreted. In still, or nearly still water, the angle of supply 

 and the diameter of the circle of supply measure the chance 

 that some convection current or drift will carry away that water. 



Text-fig. 4. 



useless for life, before the slow eddy of return brings it down to 

 the plane of the ingoing current. According to the distance 

 to which it is so carried is the percentage of clean, unused water 

 which enters the organism, and according to this percentage 

 is the chance of life of the organism ; and in a slow tidal channel 

 it is clear that the distance to which the foul water will be 

 carried by the tide before it is drawn back to the plane of the 

 ingoing currents, depends directly upon the length of tlie 

 oscular jet. 



The length of this jet was shown by the experiments to vary 

 as the initial velocity-^— a result to be expected by elementary 

 theory, though a full theory would be difficult. With jets of 

 the same initial velocity, but from oscula of different size, 



X 2 



