Porifera. 



309 



spicules of carbonate of lime, as seen in the simple sponge 

 described. These spicules have various forms, but the 

 three-rayed form is most common. They often resemble 

 crystals. 



2. The silicious sponges, with spicules of silica or 

 flint. These skeletons often appear to be made of spun 

 glass, and many of them are 



of great beauty, one of the 

 most noted being the Venus's 

 flower basket, found about 

 the Philippine Islands. 



3. The horny sponges, or 

 sponges of commerce. These 

 skeletons are composed of 

 a substance called spongin, 

 whose chemical composition 

 resembles that of silk. Its 

 fine, threadlike fibers branch 

 and interweave, forming a 

 feltlike structure, with which 



all are familiar. Its chief value consists in its absorbing 

 power, and this, in turn, depends on its softness, fineness, 

 and elasticity. Its durability is also a factor in its value. 

 Some sponges have both silicious spicules and horny fibers. 

 The Commercial Sponge at Home. If one could "call 

 upon " one of these sponges, he would find it attached to 

 rock at the bottom of a warm sea. He would see a round- 

 ish mass with a smooth exterior, in color and finish not 

 unlike a dark-colored kid glove. He might see several 

 large openings, from which currents of water are emerg- 

 ing, bearing carbon dioxid and other impurities. Over the 

 surface he might discover smaller holes, into which the 

 water flows, bearing oxygen and food. If he were to dis- 



FIG. 177. CROSS SECTION OF SIMPLE 

 TUBULAR SPONGE. 



Showing Ci) three-pointed spicules (skele- 

 ton) , (2) cilia which bring in water through 

 (3) the holes in the sides of the tube. 



