SPONGES. 177 



a. Large crater-like tubes, opening at the top of the 

 sponge. Looking into these, it may be seen that 

 they give off branches. If you can see right 

 through the sponge by looking into these open- 

 ings, you may know that too much of the base 

 has been cut away* and your specimen is not a 

 good one. With a razor or sharp knife, cut the 

 sponge in two down one of these large tubes, and 

 examine from the inside. 



b. Trace the branches of the large tubes by gently 

 pushing into them a probe (a wire with a little 

 knob on one end). These lead, usually, to holes 

 seen on the outside. 



c. Grooves on the surface of the sponge, some shal- 

 low, others already becoming enclosed by the 

 union of the tufts of fibres outside of them ; in 

 this way is formed another set of tubes (d). 



d. Tubes running parallel to the surface of the 

 sponge, whose cut-off ends may be seen near the 

 margins of the split sponge. Hold the half 

 sponge up to the light to see the radiating fibres 

 and the concentric series of holes indicating the 

 mode of growth of the sponge. 



e. Minute branches of the above tubes penetrating 

 the sponge in all directions. 



It must be borne in mind that the sponges we buy 

 are only the skeletons of sponges. In the living 

 sponge the skeleton is entirely imbedded in soft liv- 

 ing matter, and the skeleton cannot be seen on the 

 exterior ; in fact, its fibres are not very evident in a 

 section of a fresh sponge. The outside of the sponges 

 whose skeletons we buy, when alive resembles, in color 



