106 ANIMAL LIFE 



a sieve on which the food is collected, and finally out 

 again by the same or a different path. This constant 

 irrigation carries water within the reach of the various 

 tissues, so that without any blood-vessels they are yet 

 able to derive oxygen from the stream, and to empty 

 into it the debris of that slow combustion that water 

 cannot quench. This removal of the ashes of com- 

 bustion is largely assisted by the activity of minute 

 plants that infect and colour the coral and anemone. 



But growth, as well as breathing, depends on the 

 supply of oxygen, and as these inert animals grow 

 freely instead of moving actively, they utilise the 

 bounty of the surf for the spread of their colonies. 

 Sponges cover the rocks of our coasts, and each 

 of these, if cut into a hundred pieces, will grow 

 into a hundred sponges as large as the first. Ane- 

 mones coat our shores and will propagate, as well 

 as sponges, by cuttings. Corals form barriers and 

 reefs miles in extent, and build up island and mountain 

 by the growth of succeeding generations. Nor is their 

 life a short one. A sea-anemone lives twenty-five 

 years, and may live to over fifty, and corals are of 

 equal longevity. 



This long-continued growth implies a rich supply 

 of oxygen ; and in two different ways these fixed 

 animals gain a better supply than that which still 

 water contains. First they colonise the region of 

 surf. Sea-water when shaken up with air increases 

 the amount of its oxygen many fold, and from the sur- 

 face down to a depth of some twenty feet the amount 



