FORM OF A SPONGE 



309 



a larger proportion of the energy produced by the flagella is 

 transmitted to the osculum in the untaxed form of pressure. 

 The cloaca must be much wider than the osculum for the 

 cloacal current to be slow ; hence the bottle-shaped cloaca 

 with a small orifice, which led our forefathers to compare to 

 the stomach and mouth of animals the pressure-chamber 



Text-fig. 9. 



Clathrina blanca. Angle of supply 110°. 



and vent of the hydraulic organ of sponges, evolved with the 

 advantage, not of the retention and digestion of food, but of 

 the forcible removal of excreta. 



I shall not here discuss the sluggish currents of Clathrina 

 and Leucosolenia ; nor the evolution of subdermal spaces, 

 and their possible development as muscular pumps with the 

 collars acting as valves (as in the strange reversed canal 



