FORM OF A SPONGE 



301 



sufficient food-catching machine than its ancestors, so as 

 eventually to make a fixed organism which is independent of 

 chance currents from waves and tides. 



Watching under the microscope a fiagellum with such a 

 rapid period of vibration that the eye only sees a mist 



Text-fig. 8. 



Wall of flagellate chamber, with two afl^erent joores. 



terminated by the two extreme positions, we are apt to think, 

 and to say, that the fiagellum is moving rapidly ; and much 

 theory has been written about the mechanics of the collar- 

 cells and choanoflagellates, on the assumption that food- 

 particles are thrown through the water at express-train speed 

 by this rapidly moving fiagellum, as a savage hurls a projectile 

 with a throwing-stick, or as civilized man drives a ball with a club. 



