USES OF ALGA 39 
land. Gracilaria spinosa is used by birds, allied to the swallows, 
for making their nests—the edible nests found in large numbers 
on the islands of the Indian Archipelago, especially in the caves 
on the shores of Java, and gathered and sent to China, where 
they bring large prices and are used in making the famous birds’- 
nest soup. Gracilaria lichenoides, also a species of the Eastern seas, 
is the source of agar-agar, a preparation used in laboratories as a 
culture-medium for.bacteria. Fossil diatoms are ground and 
used for polishing-powders. Seaweeds are everywhere used by 
farmers on the coasts as fertilizers. 
