42 



SPONGES 



composed of silica; in others, the skeleton is formed from 

 spicules composed of lime carbonate; while in, others, it 

 is formed from fine, flexible fibers of a substance called 

 spongin, which is allied to silk in chemical composition. 

 Those sponges which have a skeleton composed of silica are 

 harsh and unfit for domestic purposes. Those with skele- 

 tons of spongin are soft and are used for various domestic 

 purposes. See Figure 14 for spicules of various shapes. 



Siliceous sponges. — The most beautiful of marine 

 sponges have skeletons composed of sihca. The spicules 



are like spun glass, 

 and in the sponge, 

 known as the Venus's 

 basket, they are ar- 

 ranged so uniformly 

 that the skeleton re- 

 sembles a piece of 

 fine lacework (Fig. 

 15, E). This sponge 

 is found growing 

 near the Philippines 

 in about ten fathoms 

 of water. 



Another one, some- 

 times called the glass- 

 rope sponge (Fig. 15, 

 H), has a long stem 

 composed of the long, 

 white, glassy spicules 

 twisted together. 

 The sponge body grows on the upper end of this stem, 

 while the lower end of the stem is anchored in the mud. 



Fig. 15. 



- E, Venus's basket sponge ; H, glass- 

 rope sponge. 



