62 HALF AN HOUR WITH SPONGES. 



lime for the purpose of living on the animal matter 

 contained within the shell, just as a dog will crunch 

 bones for the sake of their organic matter. That it 

 is a true sponge, however, you may discover for 

 yourself, if you will be at the trouble of boiling the 

 yellow-looking substance in dilute nitric acid, as the 

 result will furnish you with an abundance of pin- 

 Fig. 24. 



Spicules of sponge (Glione celata), magnified. 



shaped spicules, like those shown in Fig. 24. Boring 

 sponges must have been in existence for a long time, 

 as we find the thick limy ossicles of Belemnites, 

 fossilised in the chalk, perforated in a similar 

 manner. The thick shells of the Tertiary strata 

 indicate that the genus has been continuously in 

 existence since then. 



