SPONGIDA. 



489 



kinds (Fig. 737) secrete a siliceous 

 skeleton ; and most if not all kinds 

 secrete and thus form siliceous spic- 

 ules (Figs. 736, 738). 



FIG. 736. 



Sponge showing Siliceous Spicules. Much 

 magnified. 



Fn. 73H. 



Sponge show-ing Spicules. Much magnifier! 

 Fir,. 73!). FIG. 740. 



Sponge. 21* Living Sponge. 



