236 OUT OF DOORS. 



useful piece of knowledge to those who make their 

 living by breeding this agreeable mollusc. Still, 

 though we have found the mouth and ascertained the 

 food, we have not yet discovered the method by which 

 the food gets into the mouth. 



It is found by investigation of the substances which 

 are taken from the interior of the oyster that its food 

 consists of the minute animal and vegetable organisms 

 with which the water of the sea is thickly charged. 



If a living oyster be placed in water, and watched 

 while its valves are open, a continuous current is seen 

 to run through the shells, always passing in the same 

 direction, i.e. from right to left (taking the flat shell 

 as the upper one), and running between the gill mem- 

 branes. On examining the dark edges of the beard, or 

 gill membranes, we shall find that they are divided 

 into tiny filaments, and that each of these filaments is 

 covered with a myriad of the minutest imaginable 

 fibres, each of which is continually whirling with a 

 partially spiral movement, and producing an effect to 

 the eye as if successive waves were rolling along the 

 surface. A similar effect may be seen when the wind 

 rushes over a corn-field, and produces successive waves 

 which seem to advance rapidly, though each corn-blade 

 remains in its place. 



By the united action of the countless hosts of these 

 fibres, technically called ' cilia,' the water is forced to 

 sweep along in one uniform direction, and, being driven 

 between the two gill membranes, is obliged to pass 



