THE INFLUENCE OF ANIMAL LIFE ON LIME. 91 



will show you. The sponge, as you know, not 

 having the power of moving from place to place, 

 is fixed and motionless as the rock on which it 

 grows. Now, as the sponge is productive, that 

 is, that it propagates other sponges, it may be 

 asked by what means this is done ? The parent 

 sponge produces a large quantity of seeds, called 

 gemmules, which may be compared to very 

 small pins' heads. If these are put in a watch- 

 glass containing a little sea-water, it may be 

 seen by the naked eye that they are able to swim 

 about with great facility and quickness. On 

 examining them with a good microscope, the 

 way in which they move will be discovered, and 

 certainly a wonderful sight it is. Millions of 

 what may be called paddles, and all furiously at 

 work, cover the surface of these tiny atoms, and 

 all so rapid in their motion that it is almost im- 

 possible to perceive their shape. These living 

 seeds are washed out of the body of the parent 

 sponge by the currents of the sea, and in process 

 of time fix themselves on the rocks and there 

 grow into sponges, and fulfil at last the purposes 

 for which nature intended them, that of neutral* 

 izing the effect of caustic lime; and if this were 

 not done, very few fish would be taken by you 



