2l6 



PRACTICAL ZOOLOGY 



which cover their shells and take away their food supply. Oys- 

 ter culturists seek to prevent this by growing the bivalves in 

 frames which are pulled up during a rain, thus killing the sponges 

 with fresh water. 



The origin of flint is in part due to the activities of sponges. 

 It has been estimated that to extract one ounce of silicious 

 spicules at least a ton of sea water must pass through the 

 canal system of the sponge. The spicules aid in the formation 



Fig. 131. — Bringing in a load of sponges. (From Bui. U. S. Fish Com.) 



of flint, this substance being always associated with the remains 

 of sponges and other organisms having silicious skeletons. 



Of the commercial sponges, the common bath sponge, Eu- 

 spongia (Fig. 127, D), is the most important. The best bath 

 sponges come from the Mediterranean coast, Australia, the Ba- 

 hamas, Florida, and the north coast of Cuba. They are gathered 

 (Figs. 130, 131) by means of long hooks, by divers, or by dredg- 

 ing. They are allowed to decay, are washed, dried, and then 

 sent to market. 



The depletion of the sponge supply by unwise fishing has 



