Porifera. 



169 



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 shallow, others 

 already becoming inclosed by the union of the tufts of fibers 

 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 fibers 

 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 embedded in soft living matter, and the skeleton cannot 

 be seen on the exterior ; in fact, its fibers 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 and gen- 

 eral appearance, the back of a kid glove, varying from dark 

 reddish brown to almost black. The consistency of the living 

 sponge is about the same as that of beef liver. If one of these 

 live sponges be watched, a current of water is found to come out 

 of the larger holes at the top, and currents pass in through the 

 numerous smaller holes on the exterior. 



If the sponge be handled, many of the smaller holes close and 

 entirely disappear. 



In order to understand a little more clearly the structure of 

 the common sponge, and to see how the currents of water are 

 maintained, an examination of a simple sponge will be useful. 

 Our simplest sponges have no elastic skeleton composed of 

 horny fibers like those of the commercial sponge, but have little 

 needle-shaped and three-pronged spicules of limy matter. 



