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Using a probe (a wire with knob at end, or small hat pin), try to trace the canals from the pores to the cavities inside.

Do the fibers 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?

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. 26.—Bath Sponge.

Fig. 27.—Bath Sponge.

Fig. 28.—Bath Sponge.

Sponges are divided into three classes, according as their skeletons are flinty (silicious), limy (calcareous), or horny.

Some of the silicious sponges have skeletons that resemble spun glass in their delicacy. Flint is chemically nearly the same as glass. The skeleton shown in Fig. 29 is that of a glass sponge which lives near the Philippine Islands.

Fig. 29.—Skeleton of a glass sponge.

The horny sponges do not have spicules in their skeletons, as the flinty and limy sponges have, but the skeleton is composed of interweaving fibers of