Fig. 26.—Bath Sponge.
Do the fibres of the sponge appear to interlace, or join, according to any system? Do you see any fringe-like growths on the surface which show that new tubes are beginning to form? Was the sponge growing faster at the top, on the sides, or near the bottom?
Fig. 27.—Bath Sponge.
Fig. 28.—Bath Sponge.
Burn a bit of the sponge; from the odor, what would you judge of its composition? Is the inner cavity more conspicuous in a simple sponge or in a compound sponge like the bath sponge? Is the bath sponge branched or lobed? Compare a number of specimens (Figs. [26], [27], [28]) and decide whether the common sponge has a typical shape. What features do their forms possess in common?
Fig 29.—Skeleton of a glass sponge.
Sponges are divided into three classes, according as their skeletons are flinty (silicious), limy (calcareous), or horny.