=Commercial “Sponge.”=—What part of the complete animal remains in the bath sponge? _Slow growing sponges_ grow more at the top and form tall, simple, tubular or vaselike animals. _Fast growing sponges_ grow on all sides at once and form a complicated system of canals, pores, and oscula. Which of these habits of growth do you think belonged to the bath sponge? Is there a large hole in the base of your specimen? If so, this is because the cloaca was reached in trimming the lower part where it was attached to a rock. Test the _elasticity_ of the sponge when dry and when wet by squeezing it. Is it softer when wet or dry? Is it more elastic when wet or dry? How many _oscula_ does your specimen have? How many _inhalent pores_ to a square inch? 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. [Illustration: 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? [Illustration: FIG. 27.—Bath Sponge. ] [Illustration: 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? [Illustration: FIG 29.—Skeleton of a glass 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. The _horny sponges_ do not have spicules in their skeletons, as the flinty and limy sponges have, but the skeleton is composed of interweaving fibres of _spongin_, a durable substance of the same chemical nature as silk (Figs. 30 and 31). The _limy sponges_ have skeletons made of numerous spicules of lime. The three-rayed spicule is the commonest form. [Illustration: FIG. 30.—A horny sponge. ] [Illustration: FIG. 31.—Section of horny sponge. ] The commercial sponge, seen _as it grows in the ocean_, appears as a roundish mass with a smooth, dark exterior, and having about the consistency of beef liver. Several large openings (oscula), from which the water flows, are visible on the upper surface. Smaller holes (inhalent pores—many of them so small as to be indistinguishable) are on the sides. If the sponge is disturbed, the smaller holes, and perhaps the larger ones, will close.