68 



HALF AN HOUK WITH SPONGES. 



surface inwards, a good deal more may be learned of 

 their structure than if simple spicules alone were 

 used. 



Of course all our British species are too small to 

 have any economic value. But they are well worthy 

 of study, and will throw much light on the history 

 of the lower forms of life. Sponges have served a 



Fig. 30. 



Spicules of Haliclkondria panicea. 



wonderful part in the scheme of creation. By the 

 decomposition of their protoplasm, or sarcode, 

 chemical changes have been naturally promoted 

 which have had very important results. The sili- 

 cates of soda held in solution by sea-water have been 

 precipitated by such chemical action, and the result 

 has been the formation of bands and nodules of flint, 



