308 G. p. BIDDER 



L e u c a n d r a is comparable to the tube-nozzle on the hose. It 

 means that pressure is transmitted from the flagella through 

 the water of the canals, the water moving so slowly that loss 

 in friction still leaves enough energy at the osculum to make 

 a strong stream. As with the hose-pipe, were we to close the 

 osculum more, there would be less energy lost, and the velocity 

 at the osculum would rise, with a pin-point hole, from 8| cm. 

 to 13 cm. a second, but the tiny jet could barely travel 1| cm. 

 instead of 24 cm., while the quantity would not be a hundredth 

 of that necessary for nourishment. With the osculum half its 

 existing diameter, the velocity would be increased to 11 cm. 

 instead of 8| cm., but the length of the jet reduced to three- 

 quarters and the quantity of water to only one-third what we 

 now find them. On the other hand, were the same Leucandra 

 shaped like a cornucopia, with the widest part of its cloaca as 

 osculum, the quantity of water passing would be increased by 

 one-sixth of the existing quantity, but the velocity would be 

 only 1-| cm. a second, and the jet consequently less than half the 

 present length. For a given pressure, acting through a given 

 length of channel of fixed width, there is an optimum value 

 for the size of the osculum (as we have all found with the 

 garden-hose) above and below which it will not carry so far ; 

 with measurements of the flagellar pressure, and of the number 

 and dimensions of the canals, an equation can be made to 

 determine this optimum value. I have calculated it for the 

 Leucandra of Text-fig. 1 (Note 5), but my computations of- 

 the canals and currents are not close enough to say more than 

 that the theoretically best diameter for the osculum of this 

 specimen is 2-6 mm. ± J mm. ; the preserved diameter being 

 2 mm. Eough computations for other specimens, and for 

 a S y c n, confirmed the conclusion that the osculum is always 

 at any rate near to the optimum size for producing the greatest 

 diameter of supply ; and that this is the explanation of the 

 small and definite oscula Avhich we have all noted as charac- 

 teristic of the majority of sponges with high canal system. 



The secret of the repeated development of this common 

 type of sponge is the reduction of internal velocities, so that 



