302 FRESH-WATER BIOLOGY 



the currents .that traverse the canals of the body. While the 

 action of these flagella is invisible to the unaided eye, their effect 

 may be seen if some finely divided carmine is added to the water. 

 The particles are sucked into the little pores over the surface and 

 after long wandering, having proved indigestible, are ejected from 

 the larger orifices. 



The skeleton of these siliceous sponges, the only part that can be 

 easily preserved for study, is composed of spicules or little needles 

 of hydra ted silica (opal), averaging about one one-hundredth of an 

 inch in length, fasciculated or bound together side by side, but break- 

 ing joints, to form threads of considerable thickness along the princi- 

 pal lines of the sponge growth but thinner in the connecting links 

 that make the interspaces. The binding material along these 

 threads is not strong and its composition is not certainly known. 

 The terminal spicules projecting around the sponge uphold the 

 filmy dermis a little above the firmer body of the sponge. Where 

 the larger channels unite to form the efferent osteoles the out- 

 flowing currents stretch this dermis into little cylindrical tubes or 

 towers, technically called chimneys, with terminal openings through 

 which one may often see rejected particles shot out as from the 

 crater of a volcano. 



A few fresh-water sponges in some situations seem to be essen- 

 tially perennial ; others die in hot countries at the onset of the sum- 

 mer season, or among us at the coming of winter, or are broken 

 up by floods, floating ice, etc., so that for a season they disappear 

 from view. The ordinary annual revival of sponge life, the growth 

 after winter or after a period of desiccation, is provided for by the 

 germination of many seed-like bodies, called gemmules; these may 

 generally be found when the sponge matures, fixed as a pavement 

 layer at the base of the sponge or distributed amongst its tissues. 

 The living cells enclosed in these are protected by a firm chitinous 

 coat or shell that is again surrounded by a crust composed of 

 minute air cells, which float the gemmules and promote their distri- 

 bution to distant places. A variety of minute spicules is normally 

 found embedded in this crust as described under individual species 

 in the key. 



Whenever the favorable season arrives, that is in most regions 



