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 suljstance 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 f)f silica. The spicules 



are like spun glass, 

 and in the sponge, 

 known as the^"enus's 

 basket, they are ar- 

 ranged so unifoi-mly 

 that the skeleton re- 

 semUes a ])iece of 

 fine laceA\ork (Fig. 

 15, £"). Tins sponge 

 is found growing 

 neai- the Pliilip]jines 

 hi al)out ten fathoms 

 of A^'ater. 



Another one , some- 

 timescalled the glass- 

 rojie sponge (Fig, 15, 

 H). has a long stem 

 comjiosed of the long, 



[■IG. 1.5. Ji, Venus'.s buski-l .sponge; H. glass- wllitc, glaSSV S]liculeS 



rope sponge. twitfted together. 



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

 while tlie iowei' end of the stem is ancliored in the nnul. 



