[Illustration: FIG. 23.--EGGS and SPICULES of fresh-water sponge (enlarged).] The _egg_ of this sponge is shown in Fig. 23. It escapes from the parent sponge through the _osculum_, or large outlet. As in most sponges, the first stage after the egg is ciliated and free-swimming. =Marine Sponges.=--The _grantia_ (Fig. 24) is one of the simplest of marine sponges. What is the _shape_ of grantia? What is its length and diameter? How does the free end differ from the fixed end? Are the _spicules_ projecting from its body few or many? [Illustration: FIG. 24.--Grantia.] Where is the _osculum_, or large outlet? With what is this surrounded? The osculum opens from a central cavity called the _cloaca_. The canals from the pores lead to the cloaca. _Buds_ are sometimes seen growing out from the sponge near its base. These are young sponges formed asexually. Later they become detached from the parent sponge. [Illustration: FIG. 25.--Plan of a sponge.] =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 vase-like 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. 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? [Illustration: FIG. 26.--Bath Sponge.] [Illustration: FIG. 27.--Bath Sponge.] [Illustration: 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.