SPONGES. 



FIG. 8. FRAMEWORK OK 

 SPONGE. 



forming- a network interlaced in all direc- 

 tions ; such is the common sponge of com- 

 merce, which owes its resiliency and its 

 capability of absorbing and retaining fluids, 

 qualities which render it so useful in domes- 

 tic economy, to the construction of its horny 

 skeleton. Instead of tubes of horn, the 

 sponges usually found upon our coasts de- 

 posit in their substance crystals of pure flint, 



which vary very much in form in different kinds, while a third 

 group strengthen their framework with calcareous spicula of 



variable shape. Three different 

 kinds of sponge may, therefore, 

 grow close to each other, bathed 

 alike with the same sea-water, yet 

 they elaborate therefrom products 

 so different as horn, and flint, and 

 lime, wherewith to build a fabric 

 that supports the whole commu- 

 nity. On viewing a living sponge 

 in sea- water with care and atten- 

 tion, it is found to exhibit a con- 

 stant and energetic action, which 

 Dr. Grant's account of the disco- 

 very of this motion in a native species is very interesting. 



" I put a small branch of a Spongia coalita with some sea-water 

 into a watch-glass, under the microscope, and on reflecting the 

 light of a candle through the fluid, I soon perceived that there 



FIG. 9. FLINT CRYSTALS OF SPONGE. 



sufficiently shows its vitality. 



FIG. io. SPONGE IN ACTION. 



was some intestine motion in the opaque particles floating through 

 the water. On moving the watch-glass so as to bring one of the 



