CCELENTERATA AND PORIFERA 177 



the Bahama Islands. Seven or eight hundred craft are 

 now engaged in the sponge regions of the West Indies, 

 where the shallow water permits them to hook up the 

 animals with a 

 long-handled three- 

 pronged fork In 

 the Mediterranean 

 Sea the finest 

 sponges grow at 

 a depth of two 

 hundred feet or 

 more, and can be 

 secured only by 

 divers, many of 

 whom work with- 

 out diving suits 

 and remain under 

 water as long as 

 three minutes. The 

 deepest sponges are 

 brought up with 

 drag nets. 



The skeleton Of ^ IG " 207 -~ A salt water sponge. Photograph half 

 . natural size. 



the commercial 



sponge composed of fine spongin fibers is cleaned free of the 

 fleshy portion by means of bacteria which cause all flesh to 

 decay or rot. Some species of sponges are found growing 

 only on the backs and legs of certain crabs which are thereby 

 concealed from their enemies. The crab favors the sponge by 

 carrying it to new feeding grounds. Such mutually help- 

 ful association between animals is termed commensalism. 



