How Animals Work. 



therefore, if we watch a tube in a rock pool, we shall 

 see that it is only the head and a tuft of special organs 

 that are thrust out at the opening of the tube from 

 time to time. The gill-tufts, which form the breathing 

 apparatus of the Serpula, are all close to the head ; while 

 one of the gill filaments has become modified into a 

 long, conical-shaped, brightly tinted operculum, or 

 trap door, for closing the entrance to the tube against 

 unwelcome visitors. The effect produced, as the Ser- 

 pula cautiously pushes its head out of its tube, is rather 

 like that of a very gorgeous sweep's broom appearing 

 out of the top of a chimney. Directly the gills are 

 pushed up outside the tube, they spread out in the 

 shape of broad plume-like fans on either side of the 

 gaily tinted operculum. These plume-like organs 

 are composed of delicate bright red, slender filaments 

 placed side by side on the supporting stem, like the teeth 

 on a comb. The filaments are clothed with countless 

 waving hairs, or cilia, which are so arranged as to pro- 

 duce by their movements an upward current along one 

 side of each filament and a downward current on the 

 other side. This wonderful mechanism ensures not 

 only a constant supply of fresh sea water passing over 

 the gills so that the blood of the Serpula is kept aerated 

 and purified, but these same inward-flowing currents 

 pass down the funnel formed by the base of the fans 

 and operculum, carrying directly to the mouth of the 

 Serpula the minute animals and particles of animal 

 and plant matter upon which it feeds, and from which 

 the supply of living material for the secretion of the 

 shelly tube is partly obtained. 



A group of these worms, with their fans fully ex- 



44 



