SPONGES 



or it may be a combination of both. In any case it is the 

 hardened secretion of the soft sponge cells. The skeleton 

 of the fresh water sponge is composed of little transparent 

 needles, spicules of silica, the largest of them about one hun- 



FlG. 83. — I, Diagram of section of a fresh water 

 sponge growing on stone; arrows indicate currents 

 of water through canals and chambers ; note spicules 

 in sponge walls: A, incurrent pore or osteole; B, in- 

 current canal; C, flagellate chamber where food is 

 taken into the cells; D, excurrent canal; E, gastral 

 chamber; F, excurrent pore or osculum. 2, Micro- 

 scopic spicules greatly enlarged, within cells which 

 form them. 



dredth of an inch, straight or curved, smooth or covered with 

 brier-like points, or dumbbell-shaped and sculptured but 

 always hooked or bound together in interlacing chains. In 

 young sponges, grown on glass, bundles of them can be seen 

 projecting through the filmy outer covering (Fig. 86). 



The holes on the surface of sponges are of two sizes, small 

 incurrent pores or osteoles (Fig. 83, A) where water comes in 

 and larger excurrent oscula (Fig. 83, F) where it passes out 

 of the sponge. After water has entered the osteoles it is 

 helped onward by the briskly waving flagella in the flagellate 

 chambers (Fig. 83, C). There any food it contains is taken 

 into the sponge cells but most of it passes on through a canal 



105 



