312 G. p. BIDDER 



in the direction of the jet. So, since maintenance of oscular 

 velocity is now of no advantage, the osculum opens out com- 

 pletely to allow the maximum quantity of flow regardless of 

 oscular energy. The cloaca becomes a flat surface (Text-fig. 12), 

 and the whole sponge a disk or fan with one intake face and 

 one outflow face. If such a sponge be in really still water the 

 surrounding water will by degrees be all set into slow motion, 

 in the direction from intake to outflow, and the condition of 

 life approaches that of the deep-sea sponges and polypes. 



It is a puzzling fact at first that in most of the Hexactinellida 

 we can detect no hydraulic evolution nor hydrauhc efficiency ; 

 puzzling until we remember that in the great depths where 

 they live, an unchanging current sweeps slowly from the 

 poles to the equator. They have but to spread a net across it, 

 and, whatever their mechanical inefficiency, they have incoming 

 and outgoing streams 180° apart ; the flagella have only to 

 work the water through the many meshes formed by the feet 

 of the collar-cells. The cavity of the Hexactinelhda is no 

 pressure-chamber : it is even perforated to let the onflowing 

 water sweep out that which is befouled. Food is brought to 

 them, waste is taken away. For them in their eternal abyss, 

 with its time-like stream, there is no hurry, there is no return. 

 Such an organism becomes a mere living screen between the 

 used half of the universe and the unused half — a moment of 

 active metabolism between the unknown future and the 

 exhausted past. 



APPENDIX. 



Note 1. The Velocity of a Jet of Measured Length. 

 The empiric formula, in centimetres and seconds, 

 L =(12 ± 2) V.B. X (1 -0-023 (20-01 

 where L is the length of the oscular jet, 



V its mean velocity at the osculum, 



B the diameter of the osculum, 



t the temperature in degrees Centigrade, 



